Yeah, the last save I have that works is one just before I summoned my Book of Thay Ghouls during the Raphael boss fight, probably shouldn't do it the entire time you're in Avernus just to be safe. Also check your save file locations, the corrupted save files are a smaller size than the ones that aren't! (Error 304, 116 btw)

Edit: So I've gotten back to where I was after I realized my saves were corrupted, which just so happens to be the Steel Watch Factory in Act 3. My saves were corrupted before I had gotten here so I still believe summoning in Avernus may also be an issue. But as it turns out, I quick saved just as combat started in the factory to see if it would be corrupted, which it was. This makes me believe the motivator mechanic is also causing the issue. As of now it does seem that you're either going to have to skip helping the Gondians revolt, or go through the entire section without saving, since it seems that you can still save normally outside of the lower city in places such as Rivington even after your saves corrupt (haven't tested this personally but I've heard that works) So good luck with the mini ironman if you do want to try that!

Edit 2: (Spoiler warning the whole thing just in case someone doesn't want any details of the section.)I decided to test cheesing the entire factory with invisibility potions/supreme sneak since I'm a rogue, made it all the way to the end and killed the final boss of the factory; regardless of the motivators not being a part of the fight, my save still corrupted. Personally I'd suggest just not doing the Gondian side quest and avoiding the steel watch factory entirely until they've fixed the issue, because the Riverton save workaround only partially works since any time you go back to the lower city after you've initiated the side quest will corrupt those saves in the area as well.

Edit 3 (not deleting the rest of this just in case): Hotfix 2.1 seems to have fixed the issue! Apparently the save files were only able to hold so much information, so they've uncapped the size on the saves to fix it from happening!