That is some big temp drops that you seem to have.
Is your house very large?
Is the shower more than 100' from the water heater?
You could install tempering valves at your other fixtures.
Did the plumber pull the spindle out (stem) and check it.
Do you have well or city water?
How cold is your cold water?
Sorry to jump around with the questions, but sometimes different things cause the same problem.