From the 10th post of the second linked thread:
There are no hard and fast rules at this level, I think. Just experience reports. Get the fastest CPU you can afford and at least 16, I would say better 32 GB RAM or more. Since the above quote, some things have improved: Roon 2.0 has removed the performance limitations on macOS, however, fast Mac servers are very expensive. Windows makes sense if you are familiar with it, though some people, at least in their specific situations, reported better support with Linux, in case this is an option.
On the other hand, who knows, your NUC may work well enough once the painful ingestion is over.
ROCK is, as such, very good for the purpose and all performance is used for Roon, not for Windows background processes. The problem is that super fast NUCs don’t exist (as mentioned, maybe the SonicTransporter i9 is an option). However, this does not necessarily mean that ROCK cannot be installed on non-NUCs, there is just no guarantee. ROCK installs on non-supported hardware are called MOCK. There is a thread about it 