Best way to recover from failed Monterey upgrade episode

When updating my wife’s 2017 iMac from Catalina to Monterrey, disaster struck and it went into a 60-second cycle of rebooting, never getting to a log-in menu. If I shut it down and restarted, the cycle resumed. And I was very stupid and neglected to run a backup before starting! (What, me worry?)

And Recovery mode couldn’t reinstall Catalina (permissions error).

I finally got it to boot by holding the option key while restarting and specifically choosing Macintosh HD as the boot drive. All looks fine now, and I’m currently doing a SuperDuper backup. I’ve also manually backed up her Desktop, Pictures, Documents, etc., so in the worst case she wouldn’t lose her files.

What’s the best way to proceed from here?

To avoid whatever corruption caused the reboot cycle, I think I should erase Macintosh HD, install Catalina, then just drag her Desktop, Documents, etc. from the SuperDuper backup. (rather than a full restore from the backup). Any permissions issues I might encounter. (I’d set up the new install with her same username.)

Alternatively, now that I’ve specifically told it to boot from Macintosh HD when I restarted with the Opt key, it might restart normally. If that happens, would it be “safe” to just go on without erasing and reinstalling Catalina?

Or is there a better way?


Update: After the backup finished, I tried a normal reboot, and it didn’t work. It still requires me to hold the Option key and specifically select Macintosh HD to get to the account login window.

Next, I booted from the backup, which took >30 min., displaying “Verifying Core Sync” and “Verifying CC Library.”

So probably best to not restore all. Start with a fresh OS and just manually copy Desktop, Pictures, Documents, etc?