@BloodHoneySin Before doing anything else, I would suggest getting rid of OneDrive, or at least stopping it from syncing your Documents folder. OneDrive can complicate troubleshooting and, in the wrong circumstances, even delete files. It's easiest to deal with this while OneDrive doesn't contain any critical data, for example your saves are currently elsewhere.
This guide shows how to stop OneDrive syncing or get rid of it:
https://answers.ea.com/t5/Technical-Issues-PC/CURRENT-ISSUE-Sims-4-saves-user-content-disappearing-PC/m-p/12475370#M249772
Pick whichever category you prefer. When you've changed the settings, open Command Prompt (you can search Windows for it) and run this command:
reg query "HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders" /v "Personal"
This will return the location of Documents. If it's still in OneDrive, use this method to change it:
https://support.microsoft.com/en-us/topic/operation-to-change-a-personal-folder-location-fails-in-windows-ffb95139-6dbb-821d-27ec-62c9aaccd720
Once you've done that, let me know that it worked. You'll also need to decide whether you want your Sims 4 saves and other user data on C or on D. If C is fine, you can simply move your saves to the folder the game is reading, which it sounds like you already know how to find. If you'd rather keep the saves on D, create a symbolic link to tell the game where to look:
https://crinrict.com/blog/2020/02/moving-windows-documents-folder-to-external-drive-via-symbolic-link.html