Our bathroom looks very similar to yours. Also the situation with the sloping roof.
We have managed with two measures:
1. Roof window in the area of the bathtub to increase the standing height
2. Artificially raise the knee wall by creating a storage area (for us about 20cm)
The storage area does take up some space in the bathroom, but the gain in comfort should not be underestimated.
Hello can you perhaps explain in more detail what the overall floor plan looks like? Depending on that, the door should be positioned and then the rest of the bathroom. What kind of window is that? Sill height? Or is it a double casement window? Give us a bit more input, then there will certainly be a better solution than the one you posted.