Bathroom drainage, number of downpipes, pre-walls, etc....

Here is an alternative suggestion:

Possibly also swap the bathtub with the toilet or swap the washbasin with the bathtub.
In all combinations, you end up with two drainage pipes going down.

Thank you for this idea! We have developed and discussed what feels like 100 drafts, including yours already . This is not meant to be the topic here now ... The question for us now rather is: Does drainage always have to be vertical down to level zero? Or can drainage be horizontal (with the necessary slope) in or under the ceiling? Or something completely different? (Lifting systems are of course out of the question ).

As long as you have a slope, you can drain anywhere

The toilet is the most complicated because you need a 100mm pipe. The floor construction usually does not allow for longer distances. At most, this would still be possible in a pre-wall installation. Everything else usually gets a 50mm pipe. You can estimate yourself how much distance you can cover with the corresponding slope in your floor construction...