> humidity sensors/weather sensors/motion sensors
This was your problem. I've been using Z-Wave in my house since, I think, around 2010. I had major major issues with it at first exactly like what you describe. I "fixed" them by doing two things.
A) Banished all battery powered devices from the network. Seriously. I have one in a hall closest I haven't gotten around to getting rid of, yet. The network seems fine-ish with only that sensor but large volumes of battery powered mostly asleep devices, it falls over.
B) Abolished any notion of using my Z-Wave network for sensor reporting. No tracking humidity or weather or power with my Z-Wave network because, as you mention, the Z-Wave network completely falls over when you have volumes of sensor traffic on it.
Want to have one humidity sensor which reports hourly? Sure that's fine.
Want to have a humidity sensor in every room which reports every time there is a 1% change in humidity? lol no, your network will fall over.
My Z-Wave networks are quite large at this point, 200-300 devices, and consist of Jasco/GE switches, Jasco/GE fan controls, Jasco/GE smart outlets.
Removing sensor traffic and battery powered devices from my Z-Wave network has made it extremely reliable for me. Unscientifically I would say 99% of Z-Wave control commands execute in <1s.
