Several threads that discuss very large libraries can be found on the forum. I can’t recall that in any of those a NAS was the recommended hardware platform. Still, if you find a NAS that ticks all the marks, feel free to buy one – but I’m sure you pay more than needed for the custom OS and built-in functionality you don’t ever use when using the device just as Roon Core (and this is also true for smaller libraries). There is a reason Roon Labs offers the Nucleus(+) and ROCK.
One of the threads about very large libraries as example: