I'm having the same exact problems, except it's from level 1 to level 2. I tried to go to the castle to check out the level 2 upgrade, instant CTD. Couldn't approach from any angle close enough for it to load visible objects. Though the objects actually being rendered didn't matter. I tried approaching backwards and it still CTD'ed. I had a save that was while it was upgrading, and I can go to it just fine. Then I'll sit there, and once it finishes upgrading, bam! CTD. I can use the destroy settlement option to level it to the ground, so now it's like Schrodinger's Settlement, both fucked and unfucked. I did use Spring Cleaning to sweep up one singular pile of dust that offended me like nineteen hours ago, so it might be ye' olde scrap crash, but if that's the case, can I do anything about it, or is the only option to burn the entire settlement to the ground now?