I suggest that you repair the concrete slab (put plastic down before you pour the concrete. It will assist in keeping down dampness (if that is a problem) and support the shower base or subfloor and shower base. Test the waste line first.
I will be installing a maax shower kit. There drain is a 2" as required for this shower kit.What kind of pan are you installing? Is the drain pipe 2"----A large opening is often left for a tub---but seldom for a shower---