Re: what happens if you delete slot_00000001?
Nothing happens if you delete slot_00000001.save as it was the old auto save file. It is no longer being used and can be safely deleted.
If you have already removed the outdated mods and custom content, clear your game cache and delete the localthumbcache.package from the Sims 4 folder to help with troubleshooting.
How to Delete Cache Files
https://sims4.crinrict.com/en/2014/09/delete-cache-files/
Your game saves, families and houses will be safe and can still be found in-game.
