Stuck on second loading screen sims 3

HeyITOG22​  The Windows version of Sims 3 is more stable in general and doesn't have the extra issues introduced with the 64-bit Mac version.  Having said that, the Windows version doesn't run great on your class of hardware, so I'm not surprised you ran into various issues, and I'm not sure the experience would be better even if we could address those.

In macOS, I would try forcing windowed mode.  Open Options.ini, in Documents > Electronic Arts > The Sims 3, search for fullscreen , and set it to 0 (zero).  This helps with certain performance issues and also avoids Game Mode, which is great some of the time but can also be a problem.

Aside from that, how much RAM and free storage do your computer have?  MacOS is kind of a memory hog these days, and while it should share with Sims 3 as necessary, I can see that being an issue on an 8 GB machine.  The swapping process uses space on the drive, which is why it becomes a major problem when there isn't enough free storage.  Hopefully you have plenty since you reinstalled macOS, but I do want to confirm.  20-25 GB free should be sufficient.  And of course don't run anything alongside the game, in fact restart your computer immediately before you want to play.

If this doesn't help, please create a new admin user account on your Mac.  Give it a simple name (like Test) with no special characters or accents, and don't link it to your Apple ID.  Repair the game while in the new account (EA App > Sims 3 > Manage > Repair), then try to play online, then offline if online doesn't work.