Question: Why do you want to use a NAS as Roon Core?
If you want to use is it just for Roon, a NUC running ROCK might be a more sensible solution else you might also have to specify what else you want to use the NAS for. To me the CPU looks to weak for up to 4 active zones plus DSP – might certainly be if the NAS is used for other things too.
Should be no problem as it’s backward compatible to 1GBe. Make sure to use current network hardware (Router/Switches) if you expect 2.5GBe to work. Especially consumer grade routers may offer that speed only in certain setups and/or on specific ports.
Read also: WiFi Router Recommendations?
You should use a SSD for Roon’s database. Your 800 albums can got on to a HD. Keep in mind that Roon’s database grows with any album added to the library, no matter if it’s local or from a supported streaming service.