Your car is - let's see - something on the order of 12-18 years old, right?

Unless you can document that the coolant has been changed at least every three years with MB antifreeze, it's likely that it has a lot of deposit buildup, and if it's seen "green antifreeze" I will assure you, ABSOLUTELY, POSITIVELY that is DOES have significant deposit buildup that has significantly reduced its heat transfer capacity.

A flow test will not prove or disprove anything - only a visual inspection of the core tube interior will, which will require the end tanks to be removed and replaced, which most radiator shops cannot do on this type of aluminum matrix/plastic side tank design, however, a few shops around the country can.

Trust me, the OE cooling system is more than adequate to maintain the acceptable range of cooling system temperature, which is 80-105C, anywhere on earth, so you do not need to "re-engineer" the system. You merely need to restore the original radiator's heat transfer capacity by installing a new or competently overhauled OE spec radiator.

Stop and go driving in warm to hot weather can cause the coolant temperature to climb to 105C, which is the point that the electic fans engage at high speed. They should drop the temp to about 100C where they disengage within about 60 seconds. If not, the radiator is probably clogged up and needs replacement.

Duke