I still like installing lead shower pans ( min lead weight is 4 PSF) You can also use a Terazzo base type, but it is hard to retro fit.
The way we remove a few layers of tiles is to get a grinder and cut along the grout line about 4-8 tiles high and install a new lead pan with sides up atleast 4″-6″ high except at the entrance
A properly installed lead pan can last well over 80 years.
SylvanLMP