--- FOUND SOURCE OF BUG ---
It would seem that if you have a specified .raw file in CustomPlanets and it doesn't exist (

), then it will cause the game to freeze a few turns in. No error messages, no debug logs.
Compare this to TerrainColourSchemes, where if you specify one that doesn't exist it just uses another one instead.
Just to let the devs know. This one had me banging for a while...