Do I need the turntable in a combi microwave when using convection mode?
I hope the moderators will allow me to go off tangent and contribute my own notes of my personal experience. As per your observation, 


  "...the center of the pizza is not really cooked."


So will other foods you put in to bake in convection mode. It does not work as well as a industrial turntable oven.

So because the center of the cake will cook slower, you probably need to lower the temperature even further and hence a usual size cake may take up to 1-1/2 hours to bake.

Worse for cookies. Those that are lined near the rim of the round baking pan have their outer halves more baked than the inner halves. So at the half way mark of the baking time, I have to take them out and rotate each and every cookie 180º. Since I bake them on three levels, I also switch the bottom tray with the center one as the heat is not even for these two levels.

Lesson learned. I will get a normal oven for my kind of bakes. For anyone out there interested to get a turntable oven, make sure the one you are interested has a blast of heat directing at the center.