What Roon calls a server is the Core module and the Output module.
This should explain to you the software packages.
https://kb.roonlabs.com/Software_packages
I would not recommend installing
Roon on your Synology. It will eventually run out of gas for a large collection.
Instead just install Roon on your RPi and be done with it. Let the NAS do it's own thing.
Just install Roon Server (Core + Output) on your RPi
What you can do is place the backup folder on the NAS as a best practice.
I use three locations for the backup folder - for redundancy.
Regards