For a test mod, I have a country assigned to a custom start date with no other nations. With these options, I load up the game and it crashes with no logs? I have absolutely zero clue what is going on. I've cleared my cache, verified game files, done a bunch of stuff, but I am at a complete loss.

No logs when running in debug mode?

I doubt it will change anything, but just to eliminate the possibility, try setting your custom start date to be the default one? (by ticking the "Default" checkbox on the start date)