Sims 4 and Steam Family Sharing

vapour-nil  If you unlink your Steam and EA accounts, you'll lose access to any Sims 4 content you own in Steam.  That doesn't matter for the base game since you can claim it again, for free, in the EA App.  But any packs you own in Steam will be affected too.

An EA account can only be linked to one Steam account in its lifetime, but it can be linked to the same Steam account multiple times.  There is, however, a lockout, maybe six months, between unlinking and relinking.  I'm not sure whether it applies the first time, but at the very least, unlinking and relinking wouldn't be a sustainable strategy.

What might work, if you have no other Sims 4 content in your Steam account, is to remove the Steam copy of Sims 4 and only play through the EA App.  You could then sign into the App but not Steam when running Sims 4, and use the linked accounts for whatever other purposes you have.  Here's how to remove content from Steam:

https://answers.ea.com/t5/Technical-Issues-PC/Free-Pets-stuff-no-install/m-p/13240592#M263979

You may need to clear the EA App's cache after removing the game from Steam and before seeing it available in the App.

https://help.ea.com/en-us/help/faq/clear-cache-to-fix-problems-with-your-games/