Unfortunately a lot of the new variable-speed heat pumps require a proprietary thermostat. You can use a dumb thermostat with them, but then the unit runs at full tilt all the time, which is inefficient (more costly to run) and probably increases wear on it.

Fortunately there are some heat pumps that allow you to use any old thermostat; they figure out the right speed to run at using their own temperature sensors.

Regardless, this wasn't really the thermostat: this was a control system that monitors temperatures in each room/zone, and opens and closes the vents to hit the temperature targets in each zone. Pretty sure no dumb (or even generic smart) thermostat can handle that.

At that point it's easier to just get mini-splits for every major room in the house.

You must be American.

In Australia there are exactly zero heat pumps on the market that you can use an ecobee with. It just isn’t an option. There are no off-the-shelf options.

You don’t have to pay $1700 for this garbage $50 tablet, but it is the default that every installer will offer you without mentioning any other options (and the only options are the manufacturer’s proprietary controllers). In this case his builder procured it for him without even telling him the price. He may not even have been given a choice. You have to be extremely motivated to stay on top of every decision like this along with the thousands of other decisions involved in building.

It's funny because if that was an option people would do it, but most installers have their control system included when the install and setup the system.

You don't get asked what you want or a choice in changing it to a different device...