I got past the authorize/owner loop. I did two things differently:
- Restarted the computer after the clone, before trying to set the Startup disk
- Waited longer before trying to set the Startup disk
I think the first time I tried this (which didn't get the authentication loop), I started the clone before going to bed. But the more recent attempts, I did the clone and immediately tried to start from it. I noticed that when I did that, Spotlight was still furiously indexing the volume; perhaps that interfered.
This time it didn't ask me to set an owner, only to authorize a user account.
The restart crashed (dyld cache), so I used macOS Recovery to reinstall Ventura on the backup drive. And it worked! It booted from the backup SSD (Ventura 13.6.2, same as on internal drive), and I could start FileVault encryption -- when I got that far once before with the HD backup, FileVault gave an error.
This does make me wonder: if the authentication/owners loop was due to either needing to reboot or wait for Spotlight, what would have happened when I did the SuperDuper! Smart Update on top of the clean macOS install, if I had rebooted and waited? Same error or something else?
Anyway, next milestone will be if it survives a Smart Update. I'd think it will.