I don think building size is the issue. I have the exact same issue on Siptah with a large, but normal sized base and around 20 thralls/animals. My game crashed when leaving a vault. I have experienced crashing after leaving a vault earlier when wearing a certain set from the bountyhunters. I tried all options to restore my single player game, but looks like im out of luck. Hope this gets adressed in the next round of fixes.
More info:
Xbox series X
Crash
Single player and co-op
Siptah
Bug description
Game crashed while loading a single player save at around 60% without fail, after a crash leaving a vault. Can play exiled lands single player and online games on EL and Siptah without any problems. I play Conan using Gamepass, and bought the Siptah DLC, Nemidian DLC and architects DLC. Also used the nude DLC
Tried: reinstalling, deleting cache, deleting local save files, loading up the other single player map before loading Siptah, playing online. In earlier crashes the game loaded up after a game reset.
Reproduction
After leaving Sanctuary of the Snakemen
Had a base near the coast between the castaway camp and legendary giant spider and vault
Had around 20 different thralls and animals
Had a large treasure room of level III
Recently did my first purge
Used a lot of different building styles
Had 2 large buildings, and some smaller buildings and fortified walls
Had a bridge over a small ravine to the east, and a tower giving quick acces to the Leyshrine of the Serpent
Had 2 named level 20 thralls with me (maxed out the follower stat), using legendary enhanced warhammers, and legion armor with plates. They had gruel and some heavy loot with them
Before leaving the vault I exchanged loot with them
I was wearing a lot of loot taken from the vault
My character was wearing a dragon armor, legendary durable shield and a one handed legendary sword that gave +15% strength damage
All equipment had improvements on them
Experienced no earlier crashes while outside
Did experience earlier crashed after leaving other vaults, but could reload after using a mix of suggestions like above
Never start the game from quick resume, always remove it after playing
Added to this topic because im experiencing the same issue, hope it gets some attention and hopefully resolved. Would be sad to lose all the builds