Smart millivolt thermostat

I have an old furnace that doesn’t provide power to the thermostat. My current thermostat uses batteries. I’ve looked at many smart thermostats that I’d like, but they all require 24v. I’d like to see one that runs on batteries and millivolts. You have an outdoor cam that runs on battery power. Why not a smart thermostat?

Yes! The new doorbell will work with my old wiring and it would be great if the thermostat did too. It won’t.