Wow, I had no idea there was even demand for this kinda thing. I guess I assumed all setups were like my MQTT server that takes events from a few Zigbee devices every few minutes.
Now - realize this is how natural gas pipelines are monitored/controlled in the mountains. You want that kind of communication to be reliable and scalable right?
I started developing a system ~a decade ago and ended up writing my own messaging protocols/queue systems. It's been an uphill battle, and if there's something a little more modern in the wings I'm all ears (I'm porting over to more modern uC's so it makes sense to re-evaluate the stack I suppose...)
When you run the broker on the same device as your clients you can do some really nice things, like take messages out of the device and into your development machine. Or inject them.
Domain expertise can be a liability as well as an advantage.