FWIW after all these replies, the most knowledgeable answer I got privately on this question was from an IT guy who is also an audiophile. He wrote to me:
“Being an IT guy, I fall into the camp where I don't see how the switch can make much, if any, of a difference. TCP/IP packets are not guaranteed to arrive in order, the receiving device has to sort them out and then feed them to whatever interface is doing the work. Meaning the idea of jitter is not plausible, since there is no possible timing on out of order packets. All Ethernet switches have galvanic isolation, it's part of the spec, they use transformers on the interfaces. Of course using WiFi exacerbates this, since multiple devices are contending for the same interface, but any streamer that doesn't drop out when playing is buffering the data, which is needed so it can re-assemble the packets, again, tossing out jitter and phase noise since there is no digital audio data stream, just data the computer has to put back together.
Of course I could be wrong, but it's really hard for me to buy into. As an aside, I added a 2.5Gbit switch to my network a couple of weeks ago and my NAS, which is also the Roon server, is connected to that, I noticed no difference, other than I can push data to and from the NAS faster, which was the goal.
As long as all the data arrives in sufficient time for it to be reassembled and fed to buffer, all is well. I have 40mbps download speed on my internet, on a good day that is, it's usually closer to 30, and I stream lossless stuff on qubuz daily, at this very moment in fact and it's rare that it has to stop."