@Briona_rd When you uninstall a store item via the launcher, the .ebc file that held that content doesn't get smaller, and if you uninstall all the items that the .ebc file held, it doesn't get deleted. Uninstalling through the launcher should remove files from InstalledWorlds or Library, but that doesn't always happen. The point is, reinstalling after uninstalling often doesn't work: if the corresponding .ebc file is corrupt, or there is still a file in InstalledWorlds or Library, you're likely to have the same outcome.
It seems like the best approach here is to start with a clean user folder and rebuild your content from there. Move the entire Sims 3 folder out of Documents\Electronic Arts and onto your desktop, open the launcher to spawn a clean folder, install one small store item, and see whether it shows up in-game. If it does, quit, restart your computer, and check again.
If the content disappears, please let me know whether there's a file labeled dcdb0.ebc in DCCache (as opposed to the temp files that will also be there). Please also let me know whether you have OneDrive running.