Found a few quirks and features (bugs), I managed to get around them, but maybe it's useful for devs

Didn’t want to spam the bugtracker, because they might be all fixed.
Host OS was Ubuntu 18.04 LTS.
Virtualmin has been installed in Virtualbox and Hyper-V both, encountered bugs under both, so here it goes.

[ CRITICAL ] Installation is just simply broken with Hetzner Cloud + Floating IP. Installation of floating IP before even install is highly recommended. Dual IP is definitely NOT recommended.
[ CRITICAL ] Nameserver is borked. sudo apt install resolvconf then nano -w /etc/resolvconf/resolv.conf.d/tail, then resolvconf -u and you have a working DNS resolv. HAPPENED ON HETZNER CLOUD VPS - did not happen under Hyper-V.
[ CRITICAL ] FirewallD does not add (or re-add) network interface(s) to zones, ie.: public. Resulting in ALL open ports. FirewallD should check if ALL interfaces have a zone ALWAYS. Using the CLI or GUI to add the zones properly works. HAPPENED ON HETZNER CLOUD VPS, IN HYPER-V GUEST.
[ CRITICAL ] OpenDKIM does not run properly until you issue the following commands. Yes, you enable it in the panel, you set it up properly, but it doesn’t matter. The /etc/default/opendkim file will get ignored until these are issued:

systemctl daemon-reload
service opendkim restart

[ HIGH ] FirewallD doesn’t work in the control panel. it has to be re-enabled as boot time process, THEN it runs/works. HAPPENED ON HETZNER CLOUD VPS.
[ HIGH ] Upon copying a Let’s Encrypt cert to Dovecot/Postfix, they’ll refuse to function further until you do a reboot. I am sure restarting services work just as well, but meh.
[ NORMAL ] Low CPU speed can cause the clamav step during install to timeout. Timeout/wait time definitely should be increased here.

if DNS is borked: sudo apt install resolvconf sudo nano -w /etc/resolvconf/resolv.conf.d/tail [ nameserver nameserver nameserver ] sudo resolvconf -u