windywhale You can't transfer DLC from Steam to the EA App, so buying the packs again was the right move. But perhaps the App hasn't caught up yet. Did you try clearing its cache? Here's how:
https://help.ea.com/en-us/help/faq/clear-cache-to-fix-problems-with-your-games/
If that doesn't help, are you willing to unlink your Steam and EA accounts? You'd lose access to EA games you own in Steam, at least until you re-link the accounts, but that doesn't matter if the link is only for Sims 4 and you'll be playing on a Mac for the forseeable future. And you'll be able to link the accounts again later if you have the need.