Chasing this cooling issue for months

If you have significant flow in the upper hose when the thermostat is closed, you have an issue with the thermostat bore / housing.

With the engine cold, pull the radiator cap, and take the speed up to 1500 to 2000 rpm. If you see coolant flowing, it is bypassing the thermostat.

Unless you have been installing the thermostats upside down, there is no way water pump pressure can push one open.

What can happen is a mismatch of parts, or a poorly machined manifold / water neck allowing significant coolant flow to bypass the thermostat.

Through the years, mopar went from a 2.500 OD thermostat to a smaller one. The water neck bolt spacing changed, but I would not bet that some aftermarket parts were made with a mix of the two dimension sets...

Really the only place to get water into the radiator from the engine is the thermostat. The bypass, heater, etc all work against any flow heading to the radiator.

Check to see if you have upper hose flow on a cold start.

B.