So, my Windows 10's system files got corrupted and the only way to fix it was to do a fresh install. I backed up my saves and my plethora of mods, since its a collection I've acquired over literal years and from too many places. I also backed up the game directories from the root folder because I can't remember where my disks have gone and I'm sure some have been damaged. I knew there was a slim chance it would just work after being copied.

Spoiler, it didn't. When booting up Apartment Life I get the following error message: The Sims 2 is not installed on this system, or some required files have been deleted. This expansion pack requires The Sims 2 in order to run. Please install The Sims 2 before attempting to run this application.

Any advice on what I can do to get it back up and working?

You have to reinstall the game. Just copying over the files won't work.

You can use the saved game files from the Documents folder, though. That's the only files that work as a backup.

I guess what I mean is, is there way to fix it WITHOUT digging up my old scratched discs/buying new ones and waiting an eternity for them? Like Regedit or ISOs?

I believe ISOs should work, but you’d have to have them stored somewhere already, or you’re back to finding those disks.