What thermostats do is have upper and lower limit set points inside of the total allowed temperature range.
If the temperature is less than the low point, heater comes on until temperature reaches the high point when it turns off. The heat will get a bit more and then fall. The heater state tracks which of the 2 conditions is current, right? If it's ON then go by the upper point else go by the lower.
To tell the truth, you could build this without an MCU. I'm just not sure it'd be any easier.