@GrumpyKitten4295 I didn't realize that new saves were loading now and that that only your existing saves were affected. (I should have asked for clarification earlier, but you mentioned deleting your user folder and I put that aside initially.) In that case, if you can load a new save with your current graphics settings, then leave those settings in place. The settings wouldn't prevent you from loading an existing save without also affecting new ones.
I mentioned the potential driver from Cyberpower just to explain why I was asking, since you wondered where the third-party drivers might come from. It's still best practice, whether you're alerted of a new driver by Nvidia software or find it yourself, to download manually and run it as an admin, rather than letting GeForce Experience or anything else manage the install process. Again though, since new saves work fine, you can skip this step for now if you'd like.
To the question at hand, what happens if you try to load an existing save but a different household than you normally play? There are a few different reasons why all your old saves would be affected but not new ones. One sort of random example is if a sim in the existing household has only custom traits: the game won't know what to do with that once the traits are removed, and custom traits did break a couple patches ago. Another example is if there's a broken object on the home lots of the households you can't load, although that would probably have to be a custom object that you happened to use all the time.
Anyway, the easy test is to try loading a different household, preferably ones you've never played before now. If that works, let the game run a few sim-hours, use "save as" to rename the save, quit, reload, and try switching back to your original household. If the other househod loads fine but the original one doesn't, try evicting the sims and placing them in a different house, as a test.
If you can't load any household in a given save, please let me know the size of that save file; they're all in Documents\Electronic Arts\The Sims 4\saves.
And while I'd be happy in theory to test your saves myself, I only own about half the EPs and GPs, and even fewer SPs. So I'm not sure I could load your saves properly even if they were working for you. If all else fails, there is someone else who might have time to test, but that would take at least until tomorrow with time difference, and perhaps longer depending on how busy she is. So please test yourself in the interim and let me know how it goes.
