Have a 1 piece shower with no tub a walk in. its leaking from being old. It has spider cracks by the drain. How can I fix this without spending for a new shower

If the shower floor gives and flexes a bit when you step into it That tells me the shower was installed incorrectly. The shower floor should have been bedded in when it was installed. Is there any chance that you could open up a wall and get under the shower floor to bed it in?
We could refinish the floor but if it isn't bedded the same thing would happen again. To bed a shower floor we puddle mortar or cement under the floor and around the drain and let it cure before putting any weight on it.
Once the floor has been bedded in we can refinish the floor. Sound like a plan? Cheers, Tom