I really hope it's one of the things you've mentioned.
It at least isn't a export_buildings.txt, cuz the new buildings use the old entries. The same goes for export_descr_buildings_enums.
I've also tried to let the last building I added not be buildable by any faction, but it still CTDd.
But: When I added them to RTW vanilla, nothing was wrong at all - it just worked. This indicates that there is a limit imo, or it is something RTR based
:(
I'll try adding another hundred buildings - when it works, it's an RTR based problem. When it doesn't work, there is a limit.
I'll keep you informed