It wouldn’t be cheap, but since the cadet thermostat add on has push buttons, not a dial, you could use the SwitchBot actuator to push those buttons. This would have the advantage that it doesn’t void the warranty or require any wiring. And it wouldn’t interfere with any existing safety features. But you do need one bot per button Plus the SwitchBot mini hub for smartthings integration. And you wouldn’t know the actual thermostat settings, you would just be able to use smartthings to push buttons.
So it would obviously be better to find an actual smart thermostat, but I just thought I would mention it since it would be good for some people. (For example, I myself am a wheelchair user, so being able to use Alexa or an app to change the settings on the baseboard heater would be very useful.)
Here’s a link to a SwitchBot review I wrote for this forum about a year ago. I do use these in my own house to retrofit several dumb devices, including a one button mini blender and the eject button on my DVD player. I do understand it’s probably not what you’re looking for, but just in case. The smartthings integration is provided by the switchbot manufacturer, so no custom code is needed.
2020 Switchbot Review ( robot button pusher), integration through IFTTT or SmartThings app
