ᐅ Gypsum fiber shower area

Good evening,

I still need to waterproof my shower area.
What does the layer structure look like (connections in/out of the wall such as the mixer valve and shower hose excluded)?
- Gypsum fiberboard
- Primer
- Lastogum (gray)
- Lastogum (white)
- Flexible adhesive / tile adhesive
- Tile
- Grout

Can anyone tell me how to create a waterproof transition from gypsum fiberboard to brick/plaster wall? There is a sealing tape available, but does it also adhere well to plaster/brick walls?

My tiler (a DIYer, not a professional) would not do any special waterproofing on the brick wall. He said it has never been a problem there if you don’t do it. On the gypsum boards, however, it’s obvious that it has to be done.

Would you fully waterproof the entire bathroom? Opinions seem to differ on this. Some advise me to waterproof everything, while others recommend leaving the walls breathable where possible.
What are your experiences? I’m personally leaning toward the breathable option.
The bathroom is equipped with an exhaust fan and windows, so it can and will be ventilated thoroughly.