Hi,
You're right, not a whole lot of split screen games with open world exploration.

When googling there were some hits for the WRC games having open world freeroam, so could be worth looking into.

Also, if your PC is powerful enough to run 2 instances of Need For Speed Most wanted, it looks like you can get split screen via nucleus coop.
Nucleus coop: https://github.com/ZeroFox5866/nucleuscoop
Need for speed most wanted handler for nucleus coop: https://hub.splitscreen.me/handler/nBgGXtk5pTq9Taq2F

Dug some more.
I don't have experience wtih any of these regarding open world so please double check if they match your expectations:

WRC Games at least the latest game appear to have some kind of freeroam mode.
WRC Generations:
"There’s a lot to test, a lot of places to race, and the good old open world map is back to let you practice the game’s (still very intuitive) controls." (ref.[waytoomany.games] Sounds like it's just 1 map to practice driving on.
https://gtm.you1.cn/storesteam/app/1953520/WRC_Generations__The_FIA_WRC_Official_Game/?curator_clanid=33632095&curator_listid=39253

Not at all sure about the status of the other WRC games but wrc6-wrc10 are also 2 player split screen.

MadOut Open City (Very negative review though, haven't looked into why)
"Enjoy the good physics of cars. You can choose from more than two dozen cars, open world "
https://gtm.you1.cn/storesteam/app/428190/MadOut_Open_City/?curator_clanid=33632095

Dakar 18: Looks like open world but desert:
"Huge Cross-Country Open World: driving experience across Peru, Bolivia and Argentina with more than 12,000km2 of open world that doesn’t limit you to a track or certain roads"
https://gtm.you1.cn/storesteam/app/767390/Dakar_18/?curator_clanid=33632095

BAJA Edge of Control:
"Free roaming gameplay: drive as far as you want and find hidden tracks, jumps and more off-road madness"
https://gtm.you1.cn/storesteam/app/623090/BAJA_Edge_of_Control_HD/?curator_clanid=33632095&curator_listid=39253

Crash time 3 (From 2009)
"In a gorgeous game world, completely explorable on wheels, the player gets to solve gripping criminal cases and master other diverse assignments."
https://gtm.you1.cn/storesteam/app/33620/Crash_Time_3/?curator_clanid=33632095&curator_listid=39229

Crash Time 2 (From 2008):
"This open-world racing game features 5 huge criminal cases to solve, each with up to 10 missions to complete"
https://gtm.you1.cn/storesteam/app/11390/Crash_Time_2/?curator_clanid=33632095&curator_listid=39229

Workaround split screen with Nucleus co-op:
https://gtm.you1.cn/storesteam/app/1262560/Need_for_Speed_Most_Wanted/

Much obliged for the quick, thorough and informative replies.

A few I'd already stumbled across, but certainly going to look into one or two of these.

Thanks, again

👍
A note of caution. Even if games claim to have free roam mode and split screen mode, the free roam might be for single player only.