Now I guess I can be officially considered a Rooner - was feeling upset with work, so I felt the need for a new sunk cost in my life.
No more lurking around here without using Roon…and of course, no time-limited subscription.
Now I feel much better; thank you, Roon.
Good for you. 
So here is a first question: I obviously have to leave the Roon app open in my core headless Mac mini, right?
You can install Roon Server, which will just run the Core. Then you don’t have the UI running all the time.
You can still install Roon on the same device and open it on occasions when you do need the UI, but Roon Server will run headless on your headless Mac Mini.
If you have a computer dedicated to Roon Core, then you can install the core ony (without Roon app), and use other devices for Roon app only.
So no problem if I have already installed Roon on the core and it’s just loaded my library, right? Can I download Roon Server and install it there now?
Sorry for asking again: am I supposed to delete the main Roon app from the core if I have the Roon Server now?
Well played sir!
Yeah, you can delete it. Maybe get Roon server set up first.
Tks a lot - I have kept the Roon app just in case, but not running. One thing I have NOT understood yet:
Why do I have to re-select/re-scan the music library/folder in every endpoint outside the core? I thought this would be automatic once I select the core.
Otherwise, what is the purpose of the core?
Not sure, but I assume the rescan is happening because you have changed cores, essentially, by switching to Roon server. Not sure why it is happening for every endpoint, but I would guess it will not continue.
The rescan can take some time to complete so maybe that is what you are seeing at everyone point.
Once the scan stops, it should be stable. I’ve great success over the years using a Mac Mini as the host for Roon server. It is stable and just works, although I have to stay on top of the Mac updates.