[user removed]

Most systems have a bypass circuit that allows coolant to recirculate during warm up. The thermostat opens when the temp gets up to normal, but coolant is circulating through the engine even when the t-stat is closed.

I believe that the problem of overcapacity in cold temps should be approached
by a method that reduces the "effective capacity" of the radiator itself, which means blocking the grille to restrict airflow. This controls the capacity of the cooling system to remove heat from the engine, but would not have any effect when any situation would require that greater capacity to be utilized.

Coolant level changes are precisely relative to average system temperature. Even a 1 degree change increases the coolant level. This allows precise adjustments of air flow over the radiator to maintain that average temp within a few degrees regardless of the weather conditions.

regards
Mech