And I'm back. Whew, that was a rough upgrade. I had to do some manual PostgreSQL surgery to get it past 0.19.3 - there were two versions of the hot_rank function so the migration script was failing.
Then I noticed the logs were full of pictrs spitting out "Read corrupted data..." and crash looping, and I spent a couple of hours trying to get it to do anything. I finally had to give up and nuke it, so I guess the few images I've posted over the years are 404s now.
I started to get discouraged and I was going to just decommission the server, but I'll try to learn my lesson and keep a closer eye on it so I can catch future problems before it's too late.