@FieryFiFi To my knowledge, SimCity still doesn't run in Big Sur. I do know someone who was troubleshooting this issue a while back, but I don't think she ever found a way to make the game work. Aspyr is aware of the issue and may or may not release an update, but you can certainly post on its forums asking about it. You're far from the only one hoping to play.
One completely different option is to install an older OS on an external hard drive and boot into that when you want to play. I'm not sure how old your older Mac is, but if it can install High Sierra, that would be a good option. Booting into an external is as simple as plugging it in, restarting while holding down the option key, and choosing the drive. Installing an OS on an external drive is only a bit more complicated; the best instructions are the same as before:
https://bluebellflora.com/running-32-bit-apps-after-macos-mojave/
Sims 3 works fine in Big Sur, at least on hardware like yours, but it still has the same issues it had at release. We're all hoping there will be a patch at some point, but EA hasn't released any information either way.
By the way, it's a good idea to remove the first screenshot, the one with your specs in it, because it also lists your Mac's serial number.