Stuck on second loading screen sims 3

HeyITOG22​  To be clear, you've tried new saves in both macOS and Widnows?  Did you move any of your Sims 3 data to Windows, other than the old save you may have tested?  If so, please remove or rename the Sims 3 folder in Documents > Electronic Arts.  In fact, do this in both OSs.  The game will create a new folder when you open the launcher, one with no content, which is the point of this test.

If this doesn't help, please provide a dxdiag run from the Windows side, that is if you still have Windows installed.  Click cmd-R, enter dxdiag in the run box, wait for the scan to finish, click "Save all information," and save the file to your desktop.  From there, you can attach it to a reply using the paper clip (Attachment) icon included with the other formatting buttons.

As an aside, the command key stands in for the Windows key on a Bootcamped Mac, in case you see instructions for keyboard shortcuts you want to use.  Windows key-R opens the run box, which allows you to launch an app or other utility by typing its proper name.

I don't have windows installed anymore. When I downloaded windows I completely gave up on the old saves and just decided to start fresh. Windows was able to run the game with minimal issues. That being said, it did crash here and there which I've been used to with the Mac, but I also spent hours trying to resolve graphics and settings issues on the Windows side and those I just couldn't look past and found none of the solutions I had seen and tried worked. 

Every time I tried to change any sort of graphics or gameplay setting they would revert back to default the minute I saved and reset the game so I ended up just removing windows completely and that's when I decided to erase my disk and reinstall macOS. I have tried removing the sims 3 folder in Documents > Electronic arts a few times, resetting the EA app and my laptop in between and haven't seen any changes with that either. 

I am open to reinstalling windows if the settings thing is an easier fix than whatever is happening on the Mac side.

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.

I currently have 460 GB of storage space available and 16 GB RAM. I did try forcing windowed mode, and the issue remained the same. I made a new admin user, made sure not to connect my Apple ID, restarted my laptop, repaired the sims 3 and I also decided to remove the pets pack since I hadn't seen this issue anytime before I had downloaded it.

This time the second loading screen stopped at the same point but instead of freezing turned black. I could hear that I had clicked my way into CAS as well but I couldn't see anything. I also tried playing online and offline, online is when the screen turned black and when I tried offline it was the same was before. 

HeyITOG22​  Try disabling Advanced Rendering (if it's enabled), enabling object hiding, and turning all the graphics settings to the minimum.  Do so at the Main Menu, then quit to desktop before trying to load a save.  And when you get back to the Main Menu, make sure the changes stuck.  (I know the issue with settings reverting was in Windows, but I want to make sure.)  Start that save in Sunset Valley.

If this doesn't help, would you be open to installing an older version of macOS?  While Tahoe is compatible with the Mac I believe you have (MBP 2020, 13", i5-1038NG7... although I should really have confirmed), the OS only supports a few Intel Macs.  Plus, this particular model had issues rendering Sims 3 in the past, although I don't think I saw any reports that weren't due to other issues past Catalina/Big Sur.

Point is, it's entirely possible that the graphics driver in Tahoe works with your Mac for the most part but not in Sims 3 or with the technology it uses.  Since Sequoia is very recent, any other apps you want to use should still be compatible, but if you want to install Store or custom content through the launcher, you'll need to go back to Sonoma.  (There's an issue with the newest version of Sequoia and with Tahoe where the launcher crashes if you even view the Downloads tab.)  Given everything else you've tried, I think it might be the next logical step IF you don't have a specific reason you need to be running Tahoe.

I disabled advanced rendering and lowered all of my graphics and I was able to load into a save. I did consider installing an earlier version of macOS when I was resetting it but I figured I'd give it one more shot. I do have MBP 2020, 13", i5-1038NG7.

I'll still try to reinstall an earlier version of macOS, as I'd like to be able to play with higher graphics settings, and let you know how that works. Thanks for all your help so far. 

HeyITOG22​  If you haven't already installed an older OS, you could first try raising the graphics settings one or two at a time to see whether you can pinpoint the issue.  It might be Advanced Rendering and nothing else; the next suspects, based on other players' experiences on Intel iGPUs, would be Lighting and Tree Detail.  If you're not satisfied with the settings you need to use to keep the game stable, that would be the point to switch OSs.