@Briona_rd Scheduled scans are fine, and Malwarebytes is pretty reliable, so unless you notice that a scan runs and then your store content stops working immediately afterwards, that's not the problem.
However, something is corrupting the .ebc files or otherwise preventing them from being read properly, and I'm not sure what. The usual suspects are installing too much content at once, or installing too much into one file—the game is supposed to create a new .ebc file when the existing one reaches about 200 MB in size.
If yours are in fact larger than that, what you can try is installing into one file until it gets near 200 MB, removing that file from DCCache, and letting the game create a new one as you install more content. The older files can be put back, although you'll need to renumber them: the first file will always be dcdb0.ebc, and they need to be numbered sequentially from there.
If that's not the problem, try installing into a new Windows admin user account. You can still sign into your existing Origin and store account, but don't sync with OneDrive or import any data. If you do put together a Sims 3 folder with usable store content, you can copy it to your existing user account and try it there; even if it doesn't work, you'll still have the original in your old account.