Red Hat pretty much owns the server market so that and any derivatives will probably be first on the list.
Ubuntu seems to be more popular than it should be from a server stand point so it will be up there too.
Followed by Debian.
Expect drop off in support for distros after that would be my guess. If they don’t vary too much from Debian/Ubuntu the derivatives will probably be well supported.
I am a little sceptical on trying to support platforms too broadly - lately we have seen some rather serious bugs like the Debian BIND DNS issue, or the Apache IP assignment problem.
those were out of the box system crippling problems.
I would rather suggest one robust debian support version and one robust rhel based version.
Virtualmin 8 has support for all of the current distros. We expect to roll it out before end of year (yes, that’s a few days away). There is a dev release that Ilia has posted about a few times (I’m not suggesting you use it for production, but if you want to test it, that would be how).
A few of the packages we use have either been removed or had changes that led to some issues that slowed development a bit.