@HarvE 

I suggest you do whatever works best for your enjoyment of your Sonos system - that’s the priority here. Ideally (and if you ask tech support they will say this), wire Sonos to the router or main Eero node (assuming you don’t have dual DHCP as discussed earlier) as mentioned in the Eero article. Simpler connections are usually better.

If that location has a particularly bad environment however (the router and the Eero hub are already transmitting WiFi and the Boost would add a third transmission. Add a WiFi-capable printer or a home automation hub to the mix and there’s four or more WiFi networks), then wiring to a node instead may be better, in your case. The router, Eero Hub and Boost (and a printer, etc.) would all like to have 1m space from each other and things just might get too crowded or impractical. Of course, even if your old router is still running the network, there’s no longer a need for it to transmit WiFi so you could disable this to improve matters.

But really, as you’re using large groups, I recommend you do both. Wire the Boost to the router/hub and a speaker to one of the nodes, or the other way round - whatever fits your setup the best.

Thanks. Willing to try (almost) anything at this point, except move to a new place.

@HarvE .When you have added the Boost and updated, check in About My System that every Sonos device except the Move has WM:0 next to it,  not WM:1. For speakers connected wirelessly WM:0 indicates that the speaker is connected to SonosNet.

You may need to reboot some speakers to achieve this.

You may also need to experiment with the SonosNet channel in Settings, System, Network. 

Just thought I would report back after working with a Sonos support supervisor earlier this week.

Apparently, during idle times, the devices are ungrouping, even though the app thinks they are grouped. When I start the system after this idle period, a low level message is not getting though to regroup. A fix is to ungroup and regroup in the app, but there may be a time lag of several minutes before it happens (again, despite the app behavior).

Also learned that, when using the Wi-Fi mesh, I should designate the Port as the group coordinator since it is operates at 5 GHz. I was following the protocol from trying to troubleshoot my older SonosNet - designating a closer Play 3 as the group coordinator but it operates at 2.4 GHz. 
 

Trying to reproduce the problem again empirically and generate diagnostics for further review. Trying to stick with eero since the SonosNet problems I had (audio gapping) were more chronic. 

The SonosNet is a dedicated MeshNet, where every node acts as extender to the mesh, so it makes sense to select the closest item to the Root Bridge as group coordinator. In WiFi mode there’s no such mesh, thus keep in mind that 5 GHz frequencies, while providing faster speeds at shorter range, barely penetrate walls.

@HarvE .  I still think your determination to avoid SonosNet is misguided.  By all means, try everything else first if you wish, but if you cannot resolve your issues without it, please give it a try.

I hope that you get this sorted one way or another - I am totally convinced that that is possible.

Going back to SonosNet is still a possibility, but I will pursue this path for a bit longer. Since I posted my last message, the incident happened again, and I was able to solve it under our current theory. In contest, I never found a good fix to my SonosNet issues. 
 

I generated two sets of diagnostics - before and after my ad-hoc fix - and shared it with the support supervisor. Waiting to hear back.