Of all the migration tasks I think DNS requires the most planning and careful timing of execution. Make sure you know precisely which DNS servers are involved (registrar glue, registrar DNS servers, your DNS servers, potentially Cloudflare etc.) for updating and to reduce TTL before migration.
If you run your own slave DNS servers check carefully. When I migrated a few years ago the backup/restore did not deal with the Webmin cluster server config and it required manual setup again. Maybe that’s changed since.
And don’t forget to stop Postfix and Dovecot just before you make the backups of the old server, or you risk losing emails.
The backup of the old server can be done in one operation for all domains and VM setting from within Virtualmin, but if you don’t do this already on a regular basis, check you have enough disk space to do them in one run. If not you’ll have to backup the domains separately and move the files off the old server as each domain completes.