Re: wallpaper bug

First only put back your save game

Copy all content from Documents\Electronic Arts\The Sims 4_Old\Saves to Documents\Electronic Arts\The Sims 4\Saves 

If there's a duplicate because of the new save you created, either just overwrite the file if you don't want to keep the new save or rename the new one to a different number that hasn't been used.

Now test again. If it still works, you can put the following folders back (just copy the content like you have done for the saves to their respective folders

- Screenshots

- Recorded Videos

- Tray (your houses and households)

- Custom Music if you have any

Test again.

If it still works you can move on to your mods folder.

Copy stuff back in groups. Start with CC, most should be fine., then your mods but do make sure they are compatible

Here's the current list for broken mods/cc: https://forums.thesims.com/en_US/discussion/1009959/broken-updated-mods-cc-july-2023-horse-ranch-patch

and here's a tutorial with pics for the whole process: https://sims4.crinrict.com/eng/2014/09/resetting-the-game-to-factory-settings-without-reinstalling/