I uninstalled my Ultimate Collection because I wanted to start clean. But when I reinstalled it, everything was exactly the same as it was when I removed it. All CC was in place and everything. I feel like this would be convenient in some other circumstance, but I already have backups of everything I want to keep, and I want to start fresh (for reasons that are hard to explain and probably aren't relevant). How do I reinstall UC on the same disk after uninstalling it and have it be like a brand-new installation?

There is no need to uninstall and reinstall UC, the only thing you needed to do was delete the Sims 2 folder in My Doc and restart your game.

To start over, removing the Sims 2 folder from the My Documents directory might be a better solution. The game would produce a new one during the next start up; afterward you can restore the backups.

What if part of the reason I want to do it is because I learned that something I did may have corrupted the game files? As in, not just one neighborhood, but all future ones? Would deleting the Documents subfolder fix that too, or would it only remove all custom content and things?

game corruption; I hear there is a repair option in Origin. probably try that before trying uninstall/re-install.

There is one! I never noticed it. Hopefully that'll do the trick, then.

Start up Origin and right click on the UC icon and there will be the option to do a repair on your game.

Even so, the UC doesn't seem to wipe the files in My Documents on uninstall which is why you found everything in place when you went back.