Short history of this machine...
Runs on Catalina, customer wants to upgrade to BigSur. Crash and not possible to install no matter which OS.
Reinstalled a TimeMachine backup from Catalina on an empty SSD. Succes.
Worked well, customer doesn't do heavy tasks. A few years later, he had to upgrade to Monterey, same problem.
I did the same (install a TM backup), but once booted in OS machine does weird things, but is operative.
Let run Memtest en gives a RAM error. Look on thumbnail
https://www.badcaps.net/forum/images/attach/jp...