We want to install blinds in the two children's rooms.
That is not enough. Heat or solar heat is intercepted _before_ the window, not only when it is already inside. And if you intercept it earlier, you don't need so many expensive extra devices. It's also about light protection at night, because streets or houses are illuminated.
Is there a reason for the 3m assumption?
I think the 3 meters is simply the classic length of a wardrobe, which is cost-effective due to the standard. I also plan with that, and the attentive reader knows that I always try to plan the wall for a 3-meter wardrobe. Even though there are now Pax or other 2-meter wardrobes, classic often fits better. But the dressing table breaks the meter.
Here is a draft with a 4.5m staircase
So, there should still be space in front of the stairs to be able to enter the floor without immediately falling into a room. In addition, furniture must also find its way through the hallway upstairs. With all the optimizations: one should always check if they might possibly be deteriorations.