If you have the T6, it has HomeKit. But I think it works fine if you don't connect it to the app at all.

I don't have a heat pump but I do have a Honeywell wifi thermostat. It requires wifi to get the outdoor temperature unless it has terminals to support and install an external temperature sensor (mine did not have the terminals). Can be integrated with home assistant but through the Honeywell cloud.

Well, look, the thing has, somewhere, an io with a 'stop' and 'go' signal for each of its functions, right?

How hard would it be to copy those amd do a home made foss version off aliexpress? Maybe even put in multiple sensors and do some fanxy math?