Is an 8cm thick vertical slit possible in the outer wall?!?

Personally, I would have installed the pipe in the wall.

But that does not comply with the standard you mentioned ;-)

This is how the bathroom continues, so it would be a partition wall and not just simply covering the entire wall with drywall. The partition wall would then end after about 150-180cm.

And? I wouldn’t even come up with the idea of hiding the pipe inside the wall, since you also need space for the water pipes. Then you can nicely install a concealed fitting, etc.
100mm frame on the right, left as well as top and bottom, or as many as needed. Double boarded on that and that’s it. Or you build a shower niche (so only one door at the front). Then you can’t even see that drywall was installed on the wall.

And? I wouldn’t even consider hiding the pipe inside the wall since you still need space for the water pipes. Then you can nicely install a flush-mounted fitting, etc.
100 frame on the right, left, as well as top and bottom or as many as needed. Double-boarded on top, and that’s it. Or you build a shower niche (so only one door in front). Then you can’t even see that drywall was used on the wall.

That’s also the original idea I had already come to terms with. A pre-wall at about 150-180cm to hide all the pipes, etc.
I would just find it visually nicer if the pipes were hidden in the wall, but that doesn’t really seem practical. And when I look at the DIN you posted, such a slot for the wall thickness should only be 3cm deep, which I am almost exceeding by three times here.
It was simply the idea of my tiler that made me start thinking again.

That doesn’t match the DIN you mentioned ;-)

I would have even suspected that, hence the "personally". My lay opinion is that the DIN is interpreted quite strictly...

Ok, then the only option would probably be to go to a structural engineer, but is it worth it?!? Then I'll probably just build the stud wall...

Show the entire bathroom, including the drainage planning from the building file. With just the two individual photos, one can hardly give any well-founded input. Based on the scarce facts so far, I tend not to go beyond a gut feeling: not to change anything about the technology and to make a virtue out of the false wall: niches for bath additives / reading light above the tub etc. can actually always be useful in the bathroom.