Hello forest nymph,
You need to start from scratch and fix the sagging and the base before you even consider laying flooring.
Everything cracks, crumbles, with moisture damage and large costs as a result. Sorry to be the bearer of bad news, but improperly built wet rooms are dangerous!
If you aren't absolutely sure, you should ask a certified professional to check before you start anything.
Maybe they’ll find a simpler solution?
Best regards, Johan