At the end of the Primo review you write in the Sidebar 1: Specifications: "… HDMI (video only) … ". That means the Primo as such is not the solution if it doesn’t output audio over HDMI?!
But interesting article. I read in the review that you listen to MCh and classical music. So you solved the challenge to play MCh in a streamed environment? What gear are you actually using?
In this thread Raspberry Pi 4 HDMI Output - #23 by dabassgoesboomboom several community members report that MCh is possible with a Pi3 or 4 with either piCorePlayer or Volumio. But not for me 
as it seems.
My roon trial stops tomorrow. roon without MCh makes sense to me, as a NAD T778 would have (a not so bad) BlueOS on board. That’s at least my current impression.