UPDATE: I solved the issue above! However now that I can finally use Sims4Studio I have a ton of new issues which I'll post elsewhere...
For Mac users, the easiest way to find the route to your Sims 4 app is as follows:
1. Run the Sims 4 through the Origin app as usual
2. Once the Sims is open (loading screen is fine) hit command-tab (-tab) to switch to finder (or any other app)
3. Now you can access your dock, and you should see the Sims 4 app on there since it is currently running
4. Right click on the Sims 4 in your dock, select options > show in finder
5. A new finder window should open showing exactly where the Sims 4 app is stored, and you can trace back the route to find it from within Sims4Studio.
If you have trouble accessing the folder for any reason, here is a direct way of reaching it:
1. Go to: Mac HD > Users > [your username / home folder] > Applications
2. This folder looks identical to your regular applications folder, but the only thing in there is the Sims!
3. Right click on the Sims 4.app in there, and select 'show package contents'
4. You should now be able to look inside the 'Sims 4.app' because it's secretly a folder, not an app!!
5. Inside that folder go to: Contents > Resources > The Sims 4.app (this is the actual real app)
As you can see this is all a bit confusing, because for some reason the system has created a secret duplicate applications folder and tried to hide the Sims 4 app from you by packaging it inside a fake version of itself?? No wonder we were all stuck before! I hope this helps any other mac users who are struggling <3