Well, for certain you need to upgrade Ubuntu 16.04. That is end of life and effectively dies in a few months. (April I think is when support discontinues.)
I’m running Ubuntu 20.04 and have no issues. I don’t have any experience with Debian so I’m not sure if it’s any better or different at all. I would think any performance gain from one Linux distro to the other would be minimal at best.
If I were in your shoes, I would start fresh. I would back up your virtual servers, then wipe the drive with a low level format (I use Disc Tools for that, which is a free download) then I’d install my OS (Ubuntu, Debian, whatever you decide to go with) and then install Virtualmin via the automated script.
The Virtualmin script will automatically install everything else you need (LAMP, etc.) so no need to worry about doing that independently.
Then use the Add Server / Import Virtual Servers feature to restore your virtual servers.