I just had a similar endless loop on update (Catalina to Big Sur) last week. Apple guided me to do a T2 firmware update which bricked the machine. Sent it in fo

I just had a similar endless loop on update (Catalina to Big Sur) last week. Apple guided me to do a T2 firmware update which bricked the machine. Sent it in for service. Logic board was replaced "under warranty" (it's a 3 year old machine with no Apple Care!).