Is there a maximum thickness for shower float cement?
I am not aware of any maximum thickness requirements, only minimum. Many tile setters do not like going over a cement base that is thinner than 1 1/2" for fear of cracking, although there are additives of one sort or another to get it thinner, though not much. To have more concrete on one side or another to account for an existing condition, will not adversely affect the concrete. The concrete needs to be sloped to the drain anyway so no shower water sets.

The concern you have with a thicker concrete base is confusing to me. The only way that a thicker base can crack is to put too much water in the mix, when the water dissipates, the concrete will crack from shrinkage. But the amount of water to be used in that situation is to be only enough to get the concrete to set up. The term used with this type of concrete setting is "dry pack" at least it is what the tile setters called it. The amount of water needed to get the concrete damp enough to stay in a ball when you pick up a handful and squeeze. No squishing, no water dripping. Just getting it damp enough to hold together. With this, there is no maximum, though 2"-2 1/2" is the thickest ever needed to make the base for tile. You could add reinforcing wire to it although I have never seen it done for showers. mainly because of the rust potential, I think. Concrete and galvanized don't get along too well, let alone regular wire mesh with no rust proofing.