You need to install gnupg2 manually, as the scripts don't quite seem to get this one right then fail when adding the GPG key for the Webmin repositories.

To update the themes (manually or automatically) you also need to install git manually.

Now that Debian has moved to journalctl for the main system log files, Fail2Ban will fail to start unless you install rsyslog to re-enable the old style log files like /var/log/syslog.

Virtualmin will let you enable HTTP/2 in Apache with the Prefork MPM, however these are not compatible, if you check the main error log /var/log/apache2/error.log you will see a warning when the server starts that HTTP/2 will be disabled. Switch to the Worker or Event MPMs instead. Prefork (in my experience) was mostly used for the PHP module, however Virtualmin makes FastCGI easy to set up thus sidestepping the whole issue.