Okay, did the migration just now. Everything seems a little more responsive, but I wouldn't call it way faster.
Either way, it wasn't very scary at all. For anybody coming after me:
add postgres container to compose file like so. I named mine "postgres", added a "postgres" volume, and added it to depends_on for app and cron
run migration command from nextcloud app container like any other occ command and check admin settings/system for db state: ./occ db:convert-type --password $POSTGRES_PASSWORD --all-apps pgsql $POSTGRES_USER postgres $POSTGRES_DB
remove old "db" container and volume and all references to it from compose file and run docker compose up -d --remove-orphans