I have created some PCB’s and libraries which allow you to easily create your own custom devices that will work with Zigbee2MQTT by programming in the Arduino IDE. You could create your own sensors and control End Devices.
The VT1100 in sleep mode consumes 7uA, so can run on battery for years.
See Documentation here: https://vertorixau.github.io/
They can also be used to create your own mesh networks and you can use Node-Red to forward messages to MQTT to use any IOT platform that supports MQTT.
Please let me know if you are interested. Basically if I get enough interest I will sell them cheaply.