All of this makes perfect sense now. Thank you!
This explains your ethernet specification as well as why it differs from @patrick_mccarthy’s. I’ll use one of the four ports on my RS1221+ … I wish I had a use case for a faster network ports (or even link aggregation) but I just don’t 
I should have time to play around with this over the weekend if not sooner.
It’s a good time to move over to using Docker compose, too. It’s been more than a couple of years since I wrote this: Walkthrough : Roon in a Docker container on Synology DSM 7
Maybe I’ll update it some day.