The Sims 3 64-Bit Keeps Crashing on Mac

@nakdsims  Definitely don't buy extra storage just for Sims 3.  The questions here are whether your iCloud drive is full enough to prevent any more data from being uploaded, or whether the syncing itself is the problem.  That's why I suggested trying to play while your computer is entirely offline: no internet connection means no syncing.

For now, I'd suggest playing in Sunset Valley, Riverview, or Twinbrook, and definitely not Isla Paradiso, which is the most unstable EA world.  Keep lighting and tree detail at the minimum settings, don't add any custom content (store stuff is fine), and keep your computer offline while the game is running.  And save often.  If things are stable, you can try changing one detail at a time, for example raising a graphics setting or going back online, to see whether the crashing comes back.

If the game is still crashing even when you do all of the above, I don't really know what else to tell you except to save frequently.  The one other thing you could try is playing in a new admin account, one not tied to your Apple ID and therefore not tied to iCloud either.  There's no reason I see that it could help in your particular case, but you might get lucky.