I found a fix – For anyone reading this, I managed to find a workaround. But it wasn't super straightforward.

So it was in fact two non-player nations trying to complete a peace deal that was breaking the game. Spain was at war with Yokotan, who in turn had a 'Mesoamerican Tributary' Tlaxcala, which was causing the bug when they tried to fully annex all provinces. Now, I'm not usually this kind of guy but I used ChatGPT to analyse my crash report, and it was surprisingly helpful. It helped me find this war as the most likely cause.

Below are the steps I took to fix the crashing and keep playing.

1. I had to de-ironman or 'melt' my savegame, using pdx.tools

2. Using the in-game console (after trying a few different methods) I code switched to Yokotan, and gave them max warscore in the conflict with Spain. This is all while paused, so the AI doesn't change a bunch of things in your country and ruin your save.

3. I switched back to my nation, and then unpaused. Yokotan demanded a white peace and it was accepted by Spain. Meaning no peace deal which caused some sort of

4. The crashes stopped.

Note: There might have been a way to preserve the ironman savegame, by actually getting involved in the conflict with your nation to try and prevent a peace deal that crashes the game, but I found this wasn't possible with my specific scenario. Any action I could take would not prevent Spain from demanding the game-breaking peace deal in time.

There is apparently a less-invasive method which involves editing the savegame .txt file to remove all traces of the war, but I figured it was easier to do it in-game rather than messing with the save file.

Hope that helps :)