Installing a coolant overflow bottle is a good idea,

A tighter fit between fan tip and shroud will nearly always improve low speed cooling,
but under hard acceleration the engine’s torque will cause a twist off centerline.
You might white paint just the tip of the longest fan blade
and then attach two pieces of something soft like weatherstrip on the left and right sides of the shroud
and watch for “witness marks” of white paint on these weatherstrips to tell you the maximum fan diameter.

Mercedes OEM radiator caps are usually higher psi ratings.

Could the car’s heater core be quite old and unable to withstand more than 16 psi?