I'm not familiar with the details of setting up a Lemmy instance, but can't you already get the new instance up and running with a modified /etc/hosts instead of waiting for the DNS to be updated? That might reduce your overall downtime a bit.
Nah, part of the playbook sets up the initial Lets Encrypt certificate... so DNS needs to point to the new VPS. And the hostname needs to match the existing instance as it is referenced in the database.
Doesn't matter now anyway, I went a different route :)