Another vote for Sengled Element Classic bulbs. I have four of them, installed in table lamps, that operate on my HE Zigbee mesh network with zero issues. The Zigbee mesh works great and I've see zero negatives to having the bulbs paired to the HE directly.
The only other product I would try/trust is the Philips Hue bulbs and hub.
That might be a deal breaker for repeating outlets/plugs as well. I personally have all of my Zigbee (Sylvania) bulbs on their own HE hub and all end devices (plus 5 Samsung outlets) on a second HE hub set to a different channel. Once I did that and replaced all of the older Osram branded bulbs with Sylvania/Ledvance branded ones and updated the firmware on the bulbs (using the Lightify hub) they have been reliable. The Osram bulbs used a different zigbee controller and firmware than the Sylvania/Ledvance and I had issues with them even just repeating for bulbs. I also went with a bigger backup power supply and have had my hubs plugged into the usb ports on the front.
I the interest of full disclosure, I have recently replaced all of my Sylvania A-19 bulbs (around 33 bulbs) with Hue White Ambiance and some RGBWs. This wasnât really due to Zigbee issues, but because the Sylvania A-19 RGBWs (donât know about the tunable white bulbs) overheat and die, or overheat and lose connection with the Zigbee network (dropping any devices connected to them) and then die. The Sylvania recessed lights have not suffered from these issues in my experience and are actually my favorite lights. The Hue lights are rock solid as far as reliability goes and have a three year warranty.
I assume you don't have neutrals in your switch boxes? If that is the case, the Inovelli dimmers can be wired without them. You will also see very high praise for the Lutron Caseta which can also be wired without a neutral, but you will need the bridge to be able to use it with HE.
Yup. Jasco/GE also have a no-neutral z-wave dimmer now.
I have neutrals in all my gangboxes and still chose Caseta for 90% of my switches/dimmers. I have them everywhere except bathrooms/closets/garage. Lutron hardware is very high quality. And ClearConnect is an extremely reliable wireless protocol.
Unfortunately, it's much more complicated than just no neutrals.
From the OP's first post...
Yeah, quite more complicated than no neutrals.... No "line" or "ground" either.
The switch boxes each have a momentary rocker that switches an 18VAC feed line to the input poles of a 120VAC mechanical relay located elsewhere. Depending on which pole of the relay is activated, the relay latches into the "on" or "off" mode. The latching feature prevents the relay from always reverting to the "off" mode during a power outage. The individual relays are contained in an electrical box which also houses a total of 24 of these relays that control lighting circuits throughout the house.
Supposedly, it was the latest and greatest lighting technology of the mid-1970s. I'm just happy that the relays are heavy-duty industrial quality and are still going strong after nearly 45 years, as finding replacements would not be easy.
Have you considered swapping out these old relays with modern 'smart' relays? There are many options to choose from, which would allow you to utilize the low voltage wiring from the existing switch boxes as inputs to the new smart relay devices. I believe there are Zigbee, Z-Wave, and WiFi options available that would possibly meet the requirements. This would assume that the RF signals could penetrate the electrical box that the existing old relays are mounted in... 
Yes, I have looked into that. The last time I checked, the cost of making that switchover was actually more expensive than rewiring the entire house. Any lighting circuits that need a dimmer required a "module" that ran something like $2,000 for each dimmed circuit. That was a few years back, but there just aren't that many central lighting control systems out there that can be easily retrofit into these old relay configurations, and there aren't enough houses like this to encourage economies of scale that would make it more affordable.
If you know of any newer retrofit lighting control systems that are more affordable these days I'd be most happy to know about them.
I wasn't suggesting any nearly that costly. Instead, I was thinking you could use something like the following to essentially replace what you currently have in place...
https://www.amazon.com/Zigbee-Micro-Controller-Lamp-Module/dp/B07NBNVGV2
Those pre-Cambrian latching relays are such a PITA to work with... 
Indeed, and I have 60 of those relays to contend with 
The panels are mostly jumbled bundles of romex, in addition to all the 18V wires, though someone thought enough to label each relay and remote switch so they are a little easier to sort through.
Yeah, those are nice, and I've looked at them... the problem becomes where to put them so they are easily accessible, meet code, and don't look ugly hanging off a wall. Fishing wires through the walls is a nightmare because the builder decided to insulate all the interior walls, and instead of drywall on studs, all the walls are plywood over studs, then covered with 3/4" thick tongue and groove paneling.
I would have guessed that all of your existing 45 year old relays are already inside a large panel. If so, I was envisioning simply rewiring that panel, re-using all of the existing wiring.
I am sure everyone would love to see some pictures of this 1970's high-end lighting solution. That might really help to spark some ideas!
Wow...your house is not going to fall down. Maybe add a layer of plaster over the tongue and groove, and then some mid-60's fake oak paneling, just to finish it up? 
Oh, the existing paneling looks 70's enough as it is.
And another fun fact about the house... All the interior paneling and all the exterior siding is nominal 1" clear vertical grain western red cedar. Originally it had a cedar shake roof, as well, though that's been converted to metal, thank goodness.
If the house were ever to catch fire, by the time our volunteer fire department managed to get their boots on, the place would likely be fully involved. How it managed to survive the wildfire over Labor Day is nothing short of a miracle.
Well, I bet you donât have termites⌠or other bugs trying to get in. Itâs like living in a huge cedar chest.
A glass half-full is always welcome. 
That is what I was thinking too. You shouldn't have to change anything except the relay for the new smart device. Remove the dumb relay and put in the smart one in the same housing/cabinet.
The push button in the wall should be able to be the input to something like those dimmer modules linked above. They take 8-230V input, so 18v is perfect.
Could always do this! 
LOL!!!
That panel of switches does resemble what I'm dealing with, though mine is rather less neat in appearance, even with only 24 relays per box.
The space in the existing boxes is pretty well taken up, so it would be far easier to add a new box alongside and put the smart modules in that. I'll still need to pull some wires if I go that route, but it tends to be a little easier when simply adding to an existing circuit path up into the attic.
Sorry for taking so long to get this posted, but I just have too many other projects to juggle, and this one was shelved for a bit. Anyway, here's a photo of one of my relay boxes.
This one contains 24 relays and has dimensions of 18" X 24" x 4". I have another just like it, plus a 12-relay box as well. Of the 60 relay branches, there are about 5 that are a mystery. They don't seem to affect any lighting circuits, nor do I find any outlets that are switched by them.
And this brings me to a question about upgrading my lighting system. Is the Hubitat Elevation able to send an ascii text message to a certain IP address, based on an event like tripping a motion detector? If yes, where should I start to learn how to do this?
