Should I insulate my pizza oven between the firebrick base and concrete?
Fire brick and fire bed type refractory cement are different things both will get hot and disperse the heat. When I used to install a lot of wood stoves we poured the beds in every stove we installed, 30 years later the few stoves I have installed have used fire brick. I doubt that the oven will be hotter than coals from a wood stove and you have the additional layers below as a heat sink so for me based on what I have seen and the move to fire brick (easier install easy replace, partial replace when needed) I would go with the fire brick.
The last high efficiency stove I installed used 1” fire brick in the top to divert the air flow across pipes I did not think that would last but I did not replace them until they had been for 8-10 years so they held up with the coals and in the flame and heat of the exhaust. If really concerned create a small air gap under the brick to eliminate contact heat transfer, well significantly reduce it.