Opinion on bathroom planning

Through a double-spiral staircase, regardless of the material, you would gain significantly more space and design options, especially on the upper floor. You would be able to eliminate all these corners and narrow spots. But it comes across as if the floor plan is fixed and you have accepted the compromises you have to make.

That's true, it would greatly benefit the upper floor in terms of spaciousness, but we imagine that a double switchback staircase would take away a lot of the perceived space on the ground floor. However, we will definitely reconsider the double switchback staircase. The floor plan is not fixed, but our ideas are very set and might prevent us from recognizing obvious weaknesses. The idea is to have an open hallway on the ground floor that is not overwhelmed by a very wide double switchback staircase, so that the staircase can be accessed equally from the living area and the utility room/garage. But how could a concretely better bathroom be arranged if you gain more space on the upper floor through a double switchback staircase?

We have already seen this 70cm access to the shower and it was sufficient for us.

From 70 cm RBM, 65 cm including tiling can result.
And it's still enough for you - but what if an illness makes you or a family member a bit heavier?
These are just hints - you can sugarcoat any planning mistake - most here do that who actually don't want advice on the design.
I also somewhat forced you to post it here:oops:

But how could a concretely better bathroom be divided if you gain more space upstairs through a double quarter-turn staircase.

I would probably also favor a 2-quarter-turn staircase. Then you can structure the upper floor much better and the bathroom gets a better room.
However, we don't know the entire orientation of the house. Assuming that south is upstairs, the children's rooms already have a poor east orientation. Why don't you plan the toilets above each other? That makes more sense with the sewage.
I also wouldn't divide the bay window upstairs at all; you have it to loosen up the room a bit, but as it is, the usage seems somewhat cramped.

I also find the 70 cm wide access to the shower big enough, I have seen it like that before. Of course, there must be extra lighting above the shower. What would bother me much more are the two doors in the bathroom. I wouldn't feel like locking two doors every time, and you have to do that if you have children. The living and dining area would be too small for me. Your house's interior width is only about 10m, which is too little for me to also accommodate the kitchen. There is no space left between the sofa and the dining chair.

Best regards
Sabine