We spoke again with our site manager.
Swapping the bathtub and the toilet to save the pre-wall installation behind the bathtub does not work.
[IMG alt="1582288037977.png"]https://www.hausbau-forum.de/data/attachments/42/42586-2af32fc76157fa0365bc7ef4408d7ccb.jpg[/IMG]
Since the bathtub fittings cannot be installed in the external wall, a pre-wall installation is needed. This could be slightly narrower than the 20 cm (because of the ventilation pipe in the corner) behind the toilet, but unfortunately this does not save much space.
To have a bit more space in front of the sink, the toilet and sink could be pushed further towards the door:
If necessary, the pre-wall could then also be extended all the way to the window, giving it a clean finish.
We would probably shorten the pre-wall behind the sink a bit and end it at the height of the toilet pre-wall at the window.
An alternative would be to omit the pre-wall behind the sink altogether, since the pipes could be installed within the wall here. However, this would have the disadvantage that no shelf space would be available and no mirrored cabinet could be installed.