Barry, it’s a little unclear how your Bonn N8 is connected to the rest of your network; to your Router though the wall? Or why you need a SoTM switch at all since the UniFi has cages for SFP modules
But if anything, I would move the fibre optic link to the Bartok - where it might benefit the most - instead of the Roon Server, and just connect your Roon Server directly over CAT6 to the Unifi Dream Pro.