I bought a lifetime sub to Roon some years ago: very happy with it, with a few caveats.

1) I don't currently subscribe to Tidal, which degrades the functionality quite a lot. I have an Apple+ sub that I need for the rest of the Apple ecosystem, and a longstanding Spotify sub; I can't really justify Tidal but haven't yet convinced the kids that we can drop Spotify or Apple Music. First world problems I know. But unless your music library is vast then Tidal is very useful to get the most out of Roon. Spotify and Apple are unlikely to ever be fully incorporated into Roon's model as neither of them appear remotely interested.

2) I used to run the Core off my Mac Mini but same issues as OP. So I bought a cheap Intel NUC off one of the posters here (thanks!), and installed RoonOS. It wasn't that easy set up - RoonOS doesn't come with codecs for MP3 etc and these have to be added manually. There was a month or so of intermittently mucking about with various nerdy friends to get it up and running: if I had the right nerd handy to would have taken an hour or so but instead it was an hour of mucking about every week ...Then the NUC died (I think an SSD issue) and although I have reinstalled RoonOS it needs all the codecs reinstalling. Doh.

3) I now have three jobs, a family, and a house reno on the go. The chances of me sitting down to listen to music and browse around Roon's excellent algorithms are zip.

Ultimately, as a non-Linux type it was all bit too much effort for a simple man with too much work to do. I am tempted to just stick the Core back on a Mac and forget about it - come to think of it I have an old (2008) MacBook Pro in a drawer with a dodgy keyboard, I could just leave that on forever and connect via Screen Sharing when I have the rack installed :-) 

For those who haven't seen the light, Roon is the best multi-output platform I have ever used by a long margin (in addition to all the good music library management and algorithm stuff)

YMMV with NUCs and/or Roopie but I suspect if you are (like me) unfamiliar with the workings of Linux, your best bet is to just leave the iMac on!

b

I was sceptical of Roon but I must admit to having been converted; having the opportunity to buy a music system that will deliver great quality as opposed to a few one box solutions dotted round the house was the tipping point for me. I will still have all the other end points, but the main system will be very good and that will make listening to music more of a deliberate activity as opposed to a background thing which it has been for at least 16 or 17 years since I sold my previous hifi system before going travelling and then ending up stopping here.

At present it's running on my iMac Pro quite happily (and with 10 core Xeon, 65GB RAM and 4TB of SSD, why wouldn't it?!) but of course that means I have to leave the iMac on all the time to act as the server for the remote apps on phones and tablets.

The Nucleus is probably the most straightforward answer (and it would avoid a lot of the woes @bendud mentions above!) to avoid that or perhaps a Mac Mini running headless in the server cupboard (or as my wife incorrectly calls it, the linen cupboard) and set up to work from the iMac with screen sharing for control and maintenance. The Nucleus is $2400 though...!

Linn has free software called Kazoo that will do much of what Roon does but it's far more basic, lacks the slick GUI and is not written or maintained by software specialists. Like free camera editing software supplied with cameras, the manufacturers should stick to what they are good at. Roon is far better than Kazoo plus it works with just about every device in the house, whereas Kazoo will only work with the Linn system.

Silly question, but what is wrong with leaving the iMac on all the time?

I have used Roon for many years. I use an i5 NUC (have also used i3 fine) with ROCK installed. Works fine and is very easy to build and set up.