Hi Nadine-
Interesting post and cool to see all the folks weighing to help. Roon changed my music listening life for the reasons you mention and more. A great product when it’s working, which, fortunately, is most of the time. When I’ve needed support, my experience with their crowd sourced support model is less than stellar.
I’m late to the party, but are you doing any wireless at all? Occurs to me this might be a good diagnostic test of your hardwired network. I run Roon wirelessly on an Eero mesh network. If you could set up a temporary wireless connection to your rig you could take your internal wiring out of the loop.
The diagnostics from Roon on your server sounds like you may have a connection problem from your ISP? Speed test might not uncover that? If you still have the problem on wireless, maybe something wrong with the fiber modem?