Here’s advice on situations like yours from @brian, the CTO of RoonLabs.
TL;DR: listen to @Rugby’s advice, and consider a dedicated machine. An i7 NUC should cut it for the time being, but you might want to go MOCK if you’re going to keep on adding music. You should also be better off either with USB (in case of a NUC) or SATA (in case of a MOCK) rather than your NAS, ideally with a nightly backup of the data on your RoonServer drive to your NAS just in case.