Your Synology NAS also has an eSATA port, which is a teeny bit faster than USB 3. The following two products will be more than adequate for an external SSD solution to host your Roon database on:
One great thing about using the DS918+ is that you can access your music offsite via DS Audio on your phone. Your .m3u (non-smart) playlists will sync up with Roon as well if you keep them in a directory where your music is. I love this solution.