My preference is to use the NAS as the Server and use Volumio as a Renderer. All depends what software and equipment you have available but the NAS is designed specifically for indexing and serving files. The RPI isn’t, it’s just a multi functional compact package and fantastic at the price.
I can advise how to achieve this if you let me know
- make and model of NAS
- UI machine, pc, tablet etc. and operating system
Volumio is a project also and not a large commercial venture (like the NAS manufacturers). In my experience NAS drives are more robust to use as the Music Server. If you prefer the DIY experience Volumio is great (and I really like it).
Hope this helps