Advice on Improving a Wifi-Mesh -> Ethernet Setup?

Look into the final leg going into your streamer adding a switch that will convert the run in to fiber. As soon as any noise hit fiber it is eliminated. Small green computer co has many options.

The Shield Ground has not been well Accepted by the community, especially those who have studied Electronics. Way too many people are selling Snake Oil these days. Especially since they found out that if you make it COST the customer dearly, it will sell even better.
John Swenson Shield Ground (JSSG), a myth debunked | Audio Science Review (ASR) Forum
www.audiosciencereview.com/forum/index.php?threads/john-swenson-shield-ground-jssg-a-myth-debunked.11289/

EVS Ground Enhancers: a collective exploration of a tweak (hifizine.com)

@esarhaddon 

I think you might have nailed the answer on why I had never heard of a working ground enhancer or “Shield Ground “

Thanks. 

Speaking once again as someone who majored in Electronic Engineering and has been an Advanced class HAM radio operator for about 30 years, beyond grounding your devices (which could induce undesirable ground loops), You are more likely noticing something in your local environment that is flummoxing your signal than seeing some silly grounding improvement. I can give excellent examples of how outside influences can change your environ. A Car pulls up to the curb outside your home or even across the street. A Neighbor comes home and turns on their living room light. You put a nail on the wall to hang up a picture, and the list goes on and on. If your cables are properly manufactured such as CAT 6 or Better yet CAT7 cables of most any major brand name, you are highly unlikely going to induce a stray signal into the wires. Unless you are using one of the Braided types of speaker cables or cables that are themselves sufficiently shielded, they (the speaker cables) are much more likely to pick up induced noise. All you have to do is look at the science behind CAt 6&7 wiring. Much less other cables. Thinking about our cars, though they have far less ignition noise than they used to have, Nowadays they are huge transmitters of computer noise and even WiFi or Cell frequencies. 
What is Cat 7 Cable? - Utmel 
www.utmel.com/blog/categories/cables/what-is-cat-7-cable

The whole concept behind the OLD Cat5 braided cables for speakers was to make them impervious to INDUCED noise. The process did increase the Capacitance slightly, but when I upgraded from my 20-year-old braided cables a couple of years ago I measured them against my new CARDIS cables and the differences were negligible.

In the OLD days, it was not uncommon for radio equipment near a HAM  or CAB transmitter to get induced radiation directly through the Case. Noe even the wiring to antennas. Just one box being too close to the other. and this was in the days of STEEL cases. But in our modern times, manufacturers have designed equipment to minimize this kind of radiate interference. You are more likely to pick up the 5Ghz signal from your WiFi itself than from some outside source. And even at that, the WiFi 6 signal is weak enough that you are forced to use a MESH configuration in your home if it has more than a couple of rooms.
Wi-Fi 6 - Wikipedia
en.wikipedia.org/wiki/Wi-Fi_6#:~:text=Wi-Fi%20Generations%20%20%20%20Generation%20%20,%20%205%20%206%20more%20rows%20

Then look into where your IoT devices are positioned, or your spouse or children are playing on the Ipads and Cellio phonies. They are constantly Pinging the Local cell tower and your WiFi router. HTat is more radio radiation than almost any home would ever see from a neighbor who has a radio transmitter. The bottom line is unless you live inside a 'Faraday Cage', you will never be free of electronic noise, but commonly available off-the-shelf products are pretty well protected from this noise.

Also, your Phonograph cart and needle are the most common point of access for radio wave signals. and for the most part how well your home's overall wiring is grounded is the most important issue. I have seen more people trying to OVER-Kill the grounding thing and causing more problems than what they start with, by creating ground loops.