@leahmay74 The open water issue affects anyone playing the 64-bit version, and there's no fix for it at this point. I still have hope that EA will address the issue in a future patch, but we've had no news on that front.
As far as the crashing is concerned, the game is just as unstable as it always was, meaning that even without a hard RAM limit, it can crash for plenty of other reasons. If you're playing in IP, even a partially fixed one, you could have a stuck sim or another routing issue that suddenly takes up all the game engine's resources, or really any of a number of problems. If you're willing to use mods, NRaas ErrorTrap and Overwatch, as well as a few others, can be very helpful.
I doubt that the crashes are directly related to anything else on your computer, that is unless Origin is also crashing or iCloud decides to sync suddenly after leaving you alone for a couple hours. You could always try playing while offline though; it's not going to hurt anything. Just sign into Origin and put it in offline mode, then disconnect the internet before pressing Play.
Bypassing Origin by using TS3W.exe isn't possible in the 64-bit version, at least as far as I can see. I'm pretty sure EA blocked it, just as it did in the last update to the Windows version of the game, now five years ago. DEP settings did interfere with Sims 3 for a while in Windows installs, but DEP is not a feature of macOS.
If you want to upgrade your RAM and your Mac supports it—only a few models do—there's no reason not to. But before assuming it will help with Sims 3, you may want to check how much memory the game is using after you've been playing for an hour or two, as well as how much is left over after the system and any other apps you're running take their share. You can run the Activity Monitor (in Applications > Utilities) while you play; it doesn't take much in the way of resources, and it's easy to cmd-tab to it, or just leave it showing in a corner if you play in windowed mode.
