Well, new mac is set up and fully (!) functional, and even the 14-month-long Time Machine issue on the iMac has been solved, so no more getting into safe mode just to run a backup.
Apple Notes were the problem, from when I uploaded more than a decade of exported Evernote files. So I've lost half of those, but now have Time Machine. So, big win.
After a few continuous days of this I'd like not to look at a computer again for a while...