Any machine capable of running ROCK/RoonOS will also be capable of running a linux distro with the linux version of the Roon Server application.
I’m not sure that there is an ‘official’ recommendation on such a machine. Which is most appropriate depends upon what you want to achieve.
More important that anything else is to run Roon Server in a way that you are comfortable with. If linux scares you, stick to ROCK/RoonOS. If you are happy with Linux, then by all means run Roon Server on the linux distribution of your choice. If you have a machine with a Windows license and you are happy with Windows, you can also use that to run Roon Server (You can also run Roon Server on a Apple Mac but I will leave that out of the discussion because you have to commit to OSX as the host operating system when buying a modern Mac). The only person who can answer ‘which is right/best for me’ is the person asking 
Having said, the following gives a brief summary of what you might expect from RoonOS as installed by ROCK and a more general linux distribution (RoonOS is also linux - but linux pared right down to the bone).
ROCK/RoonOS gives the Roon Nucleus experience (plug in and go) with exceptionally little administration required (as you will have noticed with your existing setup). However, as you will have also noticed, there is very little that can be done by the user to monitor the behavior other than keeping an eye on disk/SSD utilisation (which can be done using the WebUI).
On the other hand, running Roon Server on top of linux (as with Windows or Mac OSX) allows you to use many more tools to determine what might be causing any issues but also requires more knowledge to perform the initial setup and any ongoing monitoring/administration to keep everything working well.
Running linux, Windows and Mac OSX also allows you to use the machine for other purposes as well as Roon - for example, a number here are using linux to host Roon as well as Plex and/or LMS.
In my first post, I mentioned DietPi. This is a linux distribution based on Debian and originally designed to be very lightweight targeting Raspberry Pi and similar extremely small footprint, low power devices running ARM processors. Such processors cannot run Roon Server (there is no ARM linux binary provided by Roon) although they can run Roon Bridge to form a Roon Endpoint when coupled with a USB connected DAC or an audio HAT (addon card for Raspberry Pi). However, DietPi also supports X64 which of course can run Roon Server.
The advantage of DietPi, is that it comes with a number of scripts to help with configuring the machine (setting up the network, managing disk drives/SSD, configuring SMB shares etc) and for setting up and configuring many popular applications (including, for X64, Roon Server) whilst also keeping the lightweight ethos. As a consequence, it is somewhere between RoonOS and a full linux distribution both in terms of the packages initially installed and the required knowledge to administrate it.
There is a whole thread on DietPi and how to get to grips with it. See:
For med, the only possible downside to DietPi is that it makes running the Early access version of Roon Server at bit more complicated. See: