The problem appears to be Chivalrous intentions, the mod author has recently confirmed it doesn't play well with windows 10. I stopped having problems after unsubscribing from it.

Hey thanks Rekua appreciate that

I play with chivalrious intentions and enjoy it very much, I think it's more that it doesn't play well with some of the other mods...

A warning to newcomers:

"Originally, The Old Man and the Sea mod was done in 2017 and left since then. The author Eomolch made an interesting mod that filled the seas with new water content and increased the involvement of naval units to be more relevant. However, the mod suffers greatly from issues since the main game was patched up to version 1.8 making several parts of the mod outdated and become malfunctioning. Especially since the developers added their own Water Fortress making some of the features in the original mod a bit redundant. Along with the arrival of Oyster Reef's as well, it broke the water content further, as well as breaking the continent settings in the random map generation."

There's a patch for it here: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2516928261&searchtext=old+man+and+the+sea

But the patch itself requires 9 other mods to work... :/

Yeah I tried installing that patch and its dependencies, and I'd always crash at the end of turn 1. Tried rolling back to the original mod list, and now I'm crashing to desktop every time i try to start a map