GreatAlbatross

Exactly this. The device either needs to be open enough that it can easily integrate to HA without internet access.
Or dumb enough that I can mod it with an ESP.

Anything that has to go through the manufacturer’s servers goes in the bin, the risks of data theft/rent seeking are too damned high.

I considered this, but was forbidden: The limescale would leave it looking manky.

Sounds like a job for the Forrester WRX STi.