I have both a dedicated music server (the sonicTransporter i9) and a Mac Mini in my HiFi system. When I started with Roon, I ran the Core on the Mac Mini. Since buying the sTi9, I've been running Roon Core on it and using the Mini for DSP software (HQ Player, BACCH4Mac) and music library management. Lately I've been running both Roon Core and HQP on the sTi9.
IMO, concerns about fan and electrical noise from a multi-purpose computer probably are over-blown (at least for relatively new equipment). I've never tried fibre optical connections, but am a little skeptical about significant SQ improvements. If you turn off background processes (such as Time Machine on a Mac), you can minimize the number of system events that might impact your music listening. But then, you probably can't eliminate them completely. That, and the faster processor in the sTi9, are the reasons I use the latter for Roon Core and HQ Player software. I still use the Mini to download digital music files (from HD Tracks or Qobuz) and expose its music library over the network to Roon and to the file player on my Matrix Element X (streamer+DAC+preamp).
If I were starting from scratch, I'd consider a faster M1 Mac Mini for all the functions currently spread over my 2 devices (sTi9 and Intel Mac Mini). A Nuc would work, too, but I happen to be tied into the Apple ecosystem with other devices. I probably would not seriously consider a Nucleus because IMO there are alternatives that offer more bang for the buck (faster processors, more software applications, lower cost).