rossi_ls I don't know much about troubleshooting on a Steam Deck and don't know of anyone else who does and helps with Sims 4, so I'm not sure how much the people on this site can offer. But does it make a difference whether you play with the Deck plugged in or not? How much free storage does the Deck have? And are you still running Steam OS, or did you install another operating system?
Please also test with the Deck offline, if that's an option. Normally you could put the EA App in offline mode, then disconnect from the internet before launching Sims 4, but if the App isn't usable in the normal sense, you will probably need to launch the game and then disable wifi afterwards.