So I've done a run through, and the game was fantastic.

Some notes:
weapon strategy Black hole being the only strafe primary made it all I used in walking stages. I noticed a special probably unintended feature that if you held it and used a non strafe special weapon, it would fire as if it were a strafe special weapon, even if the black hole wasn't allowed to fire at the same time.

i enjoyed the guardian legend callbacks the exactly the same functioning fish, and the final boss, though that poor final boss' probably for the best the final boss's hands weren't blocking all my shots though i've not played on harder modes so perhaps i'll be real sad

Quality of life things that I feel could be improved:
The waves on the ocean shmup level kinda bothered my eyes with the choppiness compared to the other movements. I ignored it but saw it mentioned by someone who watched the play-through so I'll bring it up.

Bullet visibility could be improved quite a bit I feel like. Especially issues where my bullets obscure enemy bullets. Prior mentioned wave's biggest weakness is that sometimes it felt like I can't see ♥♥♥♥. There was some other cases where bullet colors were pretty close to background colors. Maybe an accessibility option or something that mutes your own bullets and makes enemy bullets pretty bright colors. Mind you TGL also has this problem with individual bullets.

Other notes:
Game good, I was surprised to see that it's pretty stock SDL with low lib use. You put a lot of love into it.

I'm happy as hell you chose to do increased patterns and enemy rate for difficulty, while I'm not good enough for the hardest mode, it's still way more compelling than 'you have paper towel health and every enemy is a tank'

I'll probably throw my head at the hardest difficulty a bit, but I'm not someone who's terribly good at reading shmup patterns.