I just finished a 496-turn GS playthrough (w/o New Frontier) on Giant Earth(Full Map) w/ 12 Civs and 24 City-States. I was initially crashing to desktop around turn 260 when trying to Load the Game with an EXCEPTION_ACCESS_VIOLATION error, but I stumbled upon something which seems to have solved it: running "Load Database" under "Create Game" before trying to load the save. I don't know if this is something we were supposed to be doing the whole time, but seems to have fixed my issue.
Once I figured this out, I have managed to win the game w/ a Diplomatic Victory on turn 496. The inter-turn is still slow (there is definitely at least a minute of waiting), and loading the game takes 20-30 minutes by turn 450, but I was able to finish regardless. I disabled coastal lowland on game setup and did not start the Space Race, though I did reveal most of the map through regular exploration (IDK if this makes a difference), so these are not tested.
Another interesting note is that some models are really messed up post-260 (like some units have shields for heads), but this issue was only visual afaik. I believe that this may have been caused by one of the other mods I used being updated mid-playthrough (this could have also been why I crashed around turn 260, incidentally). The game seems to run fine otherwise.
I have no clue if any of this is helpful, but I just wanted to mention the "Load Database" thing in case it helps.