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.