Ugh. Almost nothing went well with doing migration assistant from Mojave to Monterey. I had to trash my Photo Library and recreate a new system photo library to repopulate from iCloud. I had to turn off iCloud Drive, delete the Archive versions, then turn it back on again. A bug with deleting user accounts was unrelated but took up an hour or two.
Kind of what I'm used to with Apple to be honest.
I probably would have been better off to migrate my documents folder and my mail files manually, then recreate the rest.
Unrelated but also sad: I hoped Apple's T2 to USB-C cable would let me continue to use my T2 hub and related peripherals (some Firewire 800!) but it's unstable in early testing.