I have used a Synology DS218+ as Roon Core.
This worked but quite often I faced significant performance issues and the CPU usage of the Synology was close to 100 pct most of the time.
I then switched back to my iMac (late 2015, i5) as a Core and recently bought a fanless i7 NUC which is fantastic.
If you can afford this I would recommend a dedicated NUC as a Roon Core ( depending on your library an i3 NUC for about 350 Eur might be fine, please look at the Roon Knowledge base with Roon‘s own recommendations).
If not possible your computer ( unless pre 2010 built) should work as a Core.
If you still would like to follow the „Core on NAS“ route please check Chris Rieke‘s website https://roononnas.org/en/synology/
Jens