BigSur 3.7 v126 SD runs but doesn't start copying

[gadgetdoctor]

Oh well it happened: exactly what I was afraid of. I got the Big Sur backup to work and then upgraded to Venutura.

For two weeks everything was ok but then on Dec 24 the machine went nuts and started spamming me with a message about background items added and the entire system is completely unstable and crashes 10 mins after log on.

https://piunikaweb.com/2022/12/09/ma...ication-issue/

So I just tried booting from my Big Sur backup and it gets most of the way through the startup bar but then it stops with this message:

Unrecoverable Error
SecurityAgent was unable to create requested mechanism builtinrelogin

Is there anything I can do?