Server matters in terms of sound quality even when not used as a renderer. You would be amazed home much noise the garbage power supplies and other bad parts can introduce. The core is also the more power intense function as it needs to tame a very volatile internet. The least expensive audio server will be a step up compared to a Mac Mini. Look at SGC or Roon for moderately priced options.
i can’t speak about the impact of a fiber output from a server to render.
I obviously don’t sell small green, I do sell Roon. I have never had a Nucleus returned unless someone upgraded to a better server within the 30 days.