Maybe I’m not fully aware of how variable speed works. With a smart thermostat, it will turn the furnace on or off, based on temperature. It can also use multistages (I think up to 3 with Nest) to turn it to high/med/low (I think).
But with variable speed, correct me if I’m wrong, but the fan is constantly modulating faster or slower depending on what temperature the thermostat tells it. So if a thermostat only tells it on/off, and the variable blower motor is attempting to modulate temperature on its own, wouldn’t it run into some problems when it tries to cycle down (not off) to lower the temp?