I had the same issue after SU12. After days of searching this forum I managed to piece together a fix. I needed to install the new updated Game Services in Xbox app (or Microsoft store can’t remember which). I also had a problem with my periphals which I left plugged in when I updated to SU12. Had to delete all in-game control profiles for my Honeycomb Bravo throttle which also caused the sim to hang at initial load and returning to main menu after a flight. Don’t understand it but it worked for me. None of this may help you but I know how absolutely infuriating it can be to not have the sim working for you so it’s worth throwing it out in hopes it could help someone.
I’ve also read improper USB Power Management settings in Device Manager can cause the sim to hang on loading.
i9 12900H
RTX3080
Quest2