@hillyhale Error 16 is a more generic "can't save file" issue, and it can happen for a variety of reasons, anything from the save itself being larger than 4 GB to unsupported characters in the save name to an inability to write to the user folder at all. The last one would be my first guess here since most of the other causes don't appear to apply.
Just as a test, please pause Google Drive's syncing. If this helps, you might need to pause it while you play or change how often it syncs.
If that doesn't help, please create a new admin Windows account and try playing in that. Make it a local account, as in, don't sign into Microsoft, and don't sync with any other services you normally use, Google Drive or otherwise. You won't need to reinstall Sims 3 or the EA App, but your user content won't be available. That's fine for this test; please just start a new save in Sunset Valley and see whether you can save (several times) and whether it loads properly after you restart your computer.
Thank you so much, I think you've fixed it!
Pausing the Google Drive sync didn't change anything, so I set up a new / extra admin account like you suggested, and it now seems to work absolutely fine!
It may or may not be coincidence that the new admin account has no connection with my Google Drive, but honestly I can't express how happy and grateful I am.
I've attached a picture of my Sim riding her horse as a thankyou <3
