ST Enabled floor heating thermostat solution?

You should look at apps that use remote temperature sensors to control your floor heater.

Here’s an example of one of the apps we wrote up on popular request to remotely manage homes using heaters, fans, humidifiers etc (those without thermostats). You could use the heater switches to turn on/off your floor heater. As you said you would need to purchase a temperature sensor which can be paired with ST and use that in the app to detect the floor temperature.