Position of the bathtub under a sloping roof

I would also make a shelf at the back (but probably about 20 cm) and see no problem in positioning the tub more into the roof slope.

Great, thanks! What would you consider the minimum height centered above the tub? 1.60 or 1.70 m perhaps?

I'm only 166 cm tall, so 1.6 m in front of the tub would be enough for me. But I also shower sitting in the tub. Anyone who wants to stand while showering will have a different opinion... Therefore, no one can give you a generally valid answer. What is important to you, when do you want to stand? You have a relatively steep roof, which quickly provides headroom. Try this out: attach a string at knee-wall height on the wall and run it upwards following the roof slope, then try at which height you find it good or still acceptable to step into the tub.

We will use the bathtub almost exclusively for the children. We might take a bath maybe once a year.

Here is feedback after 2.5 months in the house: We have the bathtub with 1.7 m standing height (middle of the tub) and it is more than sufficient! I am glad not to have sacrificed more bathroom floor space. It could easily have gone 30/40 cm further under the slope.