Re: Entire game shows as not installed
@Branwen84 Try running an App Recovery, which you can do even when the EA App won't open:
https://help.ea.com/en-us/help/faq/clear-cache-to-fix-problems-with-your-games/
Then uninstall and reinstall the App, and see whether it picks up your Sims 4 install. Let me know either way.
Please also make sure your Steam and EA accounts are (still) linked correctly. Open any EA website, for example ea.com, click the Sign In button, then the Steam icon, and enter your Steam credentials. You should land in the EA account linked to that Steam account, which is also the EA account you need to use when signing into the App. If you get an error when trying to sign into the EA website, let me know.