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

The shower opening should be towards the "front," so a shower partition doesn't make sense there. However, a friend came up with another idea: instead of placing the pre-wall on the side, put it at the head side. The pipe comes from above directly from the corner, so it might be possible to move the drain in the floor a bit further towards the corner. Then, a small pre-wall of 90 cm, as wide as the shower, would be sufficient. You just have to see if it's possible to relocate the drain in the floor that far.

I would avoid any possible kink in a wastewater pipe.

I would avoid any possible kink in a wastewater pipe.

Currently, however, there is also a kink at the top. I would try to chisel out at the bottom and maybe slightly shift the pipe down there. Let's see what the plumber says on Tuesday...