For application, a normal trowel is used, not a notched trowel. If you do thin-layer (e.g., with aerated concrete), you can also use something like maxit 262.
And regarding waterproofing, just extend the liquid waterproofing under the plaster. I also tiled a 20cm strip above the bathtub, then it goes into the sloping roof. From there on, it’s plaster, however, cement plaster with dispersion components. Under the plaster, I extended the waterproofing layer to about 1.60 meters. It has also proven itself when my children take a bath.