A little update. Just to be thorough I reinstalled the WD500Z-1 in another room. It works perfectly.
Here are the two scenarios.
First Install that failed:
The ST hub is installed at the top of a basement stairway. In between the hub and the location where the switch(es) failed is a wall with a sheet metal lined cavity for an HVAC return. Just to the left of that, about 2 feet, are two GE switches, which end up being between 6 and 8 feet from the hub. One dimmer, one on/off. The failed location was directly across that room, about 18 feet away. No matter what I did, which I documented above, even if I got the switches to work they would always eventually fail. So I replaced with GE switches, and they work fine.
Second install, successful.
I installed only one, the dimmer, in the kitchen. That ends up being about 10 feet from the hub, with only a closet in between. (no wire, no duct in closet). THAT location works fine. It’s also about 6 feet away from the first set of GE switches I mentioned.
Given that the two GoControl switches had LINE OF SIGHT to two other GE switches, I really don’t understand what the issue was. Even without the GE switches as repeaters, the first install of the GC’s was more than close enough to the hub.
So I’m going to store this away in my head as this. If one brand of switch has reception issues in a given location, try another brand. Yes, I’m keeping the GoControls, and will buy more. If they don’t work in a given location, I’ll just head down to Lowes and pick up a GE.
But thanks for your help JD!! It’s always fun to troubleshoot issues that have no technical rhyme or reason, lol. From a technical perspective, the only GUESS I can make is this. Due to the proximity of the existing GE switches, the hub, and the GoControls, I’m guessing the GC’s maybe couldn’t “decide” between connecting to the GE’s as repeaters, or straight to the hub. With that metal ductwork in the way, the reception would be wonky if going straight to the hub. When I replaced them with new GE’s, maybe the GE’s will attach to other GE switches as a repeater a little better than the GoControls will? That’s my best guess.