I fixed this myself.
After stopping the Syncthing Docker, I renamed the old DB folder to remove “migrated” from the end of the name. I deleted the new “v2” database folder, then I forced a docker update. This restarted the server and a new conversion of the database. It finished crrectly this time and everything came back up.
Thanks to the Syncthing team for keeping a copy of the old DB around instead of just deleting it! Very well thought out upgrade!