If your setup matches your needs, stay with it.
You can add a NUC or other PC as upgrade to your library on the DS918+ at any time should the circumstances demand it.
IMHO you can’t gain better SQ by adding unused CPU power to your Core if that’s it what your after.
From Roon Server on NAS:
What will happen if Roon Server runs on a slower CPU (such as Atom or Celeron)?
Roon Server uses a lot more CPU than any other audio software. You can read about why we have a Core and why it requires so much more CPU than other software here.
While Roon Server will work fine on these slower CPUs, consequences of running on them potentially include:
- Stuttering or dropouts in audio playback
- Slow response for searching
- Slow loading of artist, album, composer, and work pages
- Longer startup and connection times for remotes
- Slower audio analysis for normalization/crossfading/other
- Slower import of new music
Overall, the experience will not be as good as it can be. You can fix this by running on a better suited CPU, such as an Intel Core i3 or i5.