There is a guide for the insurmountable difficulty already (https://gtm.you1.cn/sharedfiles/filedetails/?id=2481736400) , but i had kinda a lot too add, that could not fit into one comment message due to the length, so here it is

GENERAL HINTS
(also - if you see the x/y notation with numbers, like 0.5\1 - it means that the perk has 2 tiers, with effect of T1/T2 respectively noted)


TRAVERSING LEVELS
- hex events always have higher priority compare to critical state events. This means you can move from hex 1 to hex 2 and will never suffer critical event from yellow or blue terrain or from resource lack (warmth, sanity, oxygen, energy) and even chain those if any events are in a line of hexes;
this red line climb part is completely safe and will not trigger any critical state events;


- before reaching the exit-hex - if you have all resources, use them to move through a dangerous terrain (yellow and blue) - to get additional experience before exiting the level (only viable if you are 5-10 hexes away from an exit);

- remember that you can escape to the base, if you get an extremely bad random - the starting point leads to the start, and there are no penalties for resetting a seed of a level;

- completing a level will change weather and base debuffs for not competed unlocked levels (this means if you dont like some levels weather combinations, you can try resetting them by completing any other level);


- it seems that critical state events have time AND\OR distance cooldown, so its more useful to climb on a steep edge with 0 energy then move in a straight line (because by this logic, the number of hexes crossed matters, not the height difference between them) - this can help adventurer with big armor from passive skills and items to tear through some final part;

- in my experience, reaching summits is much easier mission type compare to finding the anomalies, and the T3 side missions with anomalies search are even harder than the final one somehow;

ITEMS AND CONSUMABLES
- you can equip items from inventory IN THE MIDDLE of the event, when its already being triggered, which can help a lot, to swap from warmth cap to oxygen mask or safety cap. Same goes for using consumables - if you have not enough energy you can open inventory, use the item and check if its enough to perform an action - this is especially useful in case of triggering time-consuming events during the night, which otherwise can destroy your warmth bar completely;


- at least some consumables restore percentage amount of a resource, means increasing the base max value increases the effect. For example, the base value for oxygen is 100 and the adventurer can get +60 to max oxygen from his skill tree and +30 from perks, almost doubling the effect of the oxygen consumable; this is especially viable for adventurer, since scientist has its alternative passive, that clearly says that it increases the effect of consumables, and journalist has his own restoration techs (like leveling up);


- flashlight is just OP, best starter item ever in every situation, and for caving it does not even need to be equiped!

- speed debuff - if any of your resources (not health) reaches 0, you will get a debuff of not able to move fast, so running of an anventurer will only increase temperature while moving, not giving a speed bonus, same goes for the mad scientist speed buff;

- speed is a key in many cases if you have enough energy - sanity decreases with units per hour, not units per hexes, so moving in storm is sanity-heavy. Same goes for warmth at night and oxygen at death zone - the faster you move, the less you spend them; This means the green perk that gives -20% to time while high on oxygen means - more distance for safety and climbing ropes, less heat, oxygen and sanity loss, more value for other limited time perks;

- critical state events may happen even if you stay still AFTER resting, so getting full energy on 0 oxygen for example could lead to HP loss (tho its not guaranteed, just a chance);

- abilities that cost time DO NOT trigger the critical state events, so on 0 warmth you can skip night as a scientist completely with +500 exp for 4 hours and craft an item for 6 hours;

CLASS-SPECIFIC HINTS (+ BUILDS ADVICE)

The are 6 branches of climb perks - by the color - blue, brown, green, purple, yellow, red.
During 1 climb you can ONLY CHOOSE THREE of them - remember, once you picked 3rd color - other 3 will be locked till the next level.
From most viable to least: (in my opinion)

Green - oxygen-based perks +speed while oxygen is high. This one is MUST HAVE in long play runs and high mountains (3rd one is 9700- 10000 high, so it is obligatory to have). While additional 0.75 or 1.5 oxygen while moving is good, adding 30 oxygen to maximum is even better (as oxygen tank is % based consumable, you basically increase effectiveness of tanks by 30% too). And 12/20% speed buff while oxygen is over 60 let you to lose LESS sanity while moving (and on insurmountable its 2.2 per hour, so you are losing 0.44 LESS which is a lot). Also you losing less warmth as ADVENTURER, because you need at least several (2-4 hours) to boost exp with night events, losing less oxygen because moving through death zones faster.
!!!one last push perk (green) gives a great oxygen buff (+3 at all times), that allows to sleep in a death zone (with mask) for 3 days, but the debuff (-0.5 sanity on movement) - is permanent until the mission ends, so BEWARE, because taking it too early or too low on sanity can KILL you.

Brown - a long play skills. +4/7 of warmth, oxygen and energy per each sleep seems not that much, but at the same time can be exploited to generate oxygen by sleeping (at cost of time, items or resources like warmth, sanity if outside) if any other source is used. Also - as player can be affected by critical events after sleeping, this one effectively negates this probability to 0% chance, as you will always remain at 7 warmth even after the coldest night ever, even after sleeping outside AND 7 oxygen even after sleeping in death zone with 0.
Other perks have some sinergy too - under the stars makes losing 3.5/4.5 warmth less, but at cold night its still too cold to ignore. + Exp % boost at cost of 1 perk slot (so you skipping 1 level, to get more delayed levels, potentially faster - is not bad, but better to use perk glitch to take it as close to events as possible, and remember that temple events (with beacon) do not give any exp.

Purple - sanity conservation perks. Is must have at earlier levels, because most classes later get huge bonuses from items (scientist), death zone climbing (adventurer), or passive natural armor against losing sanity in events (journalist). Still pretty much must have at any EERIE map, like the final one, because it drains sanity FAST, also very important for scientists with mad genius perk and journalist meditation. The most valuable perks are time limited +3 and +5 sanity per any event, and be aware of cant stop (that gives +2 per moving and -2 per staying) - because any time consuming event can drive you crazy af fast.

Blue - speed for a time limit, then ignoring dangerous terrain for 16 and 24 hours, and analog of climbing axe but for exchange of sanity (10% for 0.2 per hour, 15% for 0.3 per hour) - usually is kinda okay branch, especially on lower difficulties, but on insurmountable usually can be sustained by scientist (with +50% to effect of herb).

Red - warmth saving - +4/6 per item usage (items without charges), 0.5/1 per hour while moving, +4 for moving but +1 energy drain while moving - is most likely optional unless you find yourself without tents, teas and have no warm closes (bad random). It can help to adjust, but alternatives are just better.

Yellow - energy saver. -18/22% for limited time, -10% in death zone, are kinda good, but I personally like to play slow with brown, which dont work good with time consuming perks. -10% energy for 10% more time is a trick, is a trap, not worth at all.


- perk glitch - if you dont like all 3 perks from leveling menu, just go back to the main menu and continue the game, this will reroll the pack, and you can pick the one you need at the exact time you need it;

As for passive abilities you have terrain, environment and event knowledge. For each level you get 2 picks, so at maximum you get 10 picks in each category.

Terrain is most simple - each level of ice snow and rock reduces energy cost by 5, 3 and 2%.
I think that most flexible 2 ice, 3 snow and 5 rock gives you ~10% energy reduction in any case is the best option, 2nd best - is to max out 2 types and ignore the 3rd.
The third option to play as safe as possible - is to 2 ice, 3 snow and 5 terrain events defense.

Environment is simple too. Only viable options are Oxygen in death zone and warmth at cold weather (not AT night!). Having 5 points in oxygen allows you to pick the oxygen debuff every climb, which is easiest one. The EERIE is kinda hard, and the time and energy consumation is THE DEATH SENTENCE. (+20% slow means 0.44 sanity, which is almost equals to basic EERIE, and having +20% to energy consummation is even worth)

Event - is most flexible. I still prefer the sanity bonuses over anything else, but lately tried more separation (like 3 energy, 3 time and 4 sanity bonuses, or 3-2-5).

Sometimes defense vs terrain is kinda helpful XD

So i finished the game 10+ times, 3+ with each character.
What i have found is -
From best to worst - Scientist, journalist, adventurer.

SCIENTIST
He is the most flexible one, the most powerful early game (like the actual difference is INSANE) and pretty good at mid and lategame.
The key to flexibility are crafting ability, that gives the access to tier 2 items right away (in most cases his level is easier than saving journalist, so you most likely go for saving him, and will have 400 exp).
With bird-eye view, exp generation for time- only and crafting, he already is BUSTED.
The climbing pick and oxygen mask are must have, and the safety hat is kinda good in extreme situations where you out of everything. Just get the take it slow (brown perks branch) and utilize caves if there are 2 or more of them close, to prepare to a climb). Ability to create consumables is a GOAT too when you out of oxygen, or need to heal, and with climbing rope you can get too 0% energy cost for a very limited time - the math behind -
10% from terrain passives
15% from climbing gloves
40% from climbing rope
30% from mad genius skill (per 3 sanity)
----already busted but can be increased
10% from purple tree in case of 60+ sanity or many of yellow branch perks (still bad tho).

Mad scientist opens 4 buffs, 3 of which are extremely useful (speed, energy, oxygen).
Speed for 3 sanity can help to spend less then 3 sanity while moving (there more you move)+ less warmth at cold and night and less oxygen in death zone, very useful in many cases;
Oxygen with T2 oxygen mask and one last push (green perk) allow to generate and save a ton of oxygen, so you will always have green 20% speed buff from a perk.

He also has skill to generate energy from dangerous terrain events (in combination with protection allows to farm energy from them and experience by just moving through them, with other classes tend to avoid as much as possible).

Also has skill to make consumables 50% more effective and special passive for long play, getting additional 10/15 energy from a sleep.


JOURNALIST
He is great character to wonder around and get a ton of experience, with and more levels in climb - means more levels in skill tree.
The passive that gives 8/12 energy per event - can grant a loop where you spend same or less energy for getting to the event, saving it for the most hard parts. Same goes for other 3 skills -

1. experience for EVERY event, even skipped one,
2. resources for leveling up (like getting some oxygen, energy, warmth, sanity and even HP per level - is great way to have even more resources if your inventory is full (still need to find balance, because you want to spend those perk levels to get bonuses to spend consumables less, so need to find some balance);
3. and less energy on any terrain. with 0.5/1 - esp with 1 - traversing flat terrain begins cost ZERO.

Journalist also has some sort of armor that lowers energy or sanity drains in events by 5/10 and 3/5 respectively. which allows always push through energy consuming consecutive triggers in 1 event.
Means if you experience -10 energy 4 times, trying to break a cage for example. journalist will spend 5*4 or 0*4 depends on the level. Crucial.


ADVENTURER
Very strange class. On normal - night build is kinda OP, but on hard and insurmountable it sucks ass. The patched version of over the edge ability, that ignores critical events is now not that broken especially because it doesnt protect from terrain critical events.
Alpine style is kinda good, but it didnt work for me on insurmountable,
What worked - is the branches of passive maximums to oxygen, then energy and warmth (maybe the last one could be skipped to get 1st alpine style tho). Passive maximums also give the same amount to effectiveness of consumables, so you get 60 % more (and with green perks for oxygen - 90% more. sic (!))
Getting Adrenaline (2.5 sanity while moving in a death zone) is essential for the final climb since its extremely big AND adventurer can boost maximum to sanity to increase effect of herbs on him.
Running combines with protective rope that allows to ignore dangerous terrain, and healing from consumables allows to consume herbs with zero harm.

Alternatively. you can try safety cap (red one), that gives passive 1.25 damage reduction, and thicc skin with similar bonus, allowing to tear and power moving in critical state for twice or even triple as much for any other character. But for me it didnt work, sadly, tho i was VERY close.

That was a long run, and I totally liked it, except maybe a part where the game just tries to kill you with succession of several bad events xD.
Hope this thing will help anyone who finds it, and i wish you to beat that damned mountain (preferably with finesse xD). Good luck have fun, climbers!