Since you went from 50/50 coolant mix to nearly straight water with chemical wetter, that has an effect like having a 30% gpm pump boost, due to the greater thermal capacity of straight water.
To me that indicates the system is “airflow limited” not liquid & pump side limited.
I would now boost airflow somehow to further test.
Start by blowing more air on the parked stationary vehicle with a large electric floor fan or leaf blower.
If this added boost of airflow lowers the liquid temperature down to the 180 degree setting of the thermostat, then try a larger diameter fan, or a same diameter fan with more blades and/or pitch.