What we do not yet know exactly is which window widths there are exactly, and therefore the widths are not yet 100% determined.
In brick, KS etc., one usually takes multiples of one eighth of a meter; with aerated concrete, multiples of 5 cm are still practical; I would not use downright "fantasy sizes." Common sizes for interior doors are 88.5 cm (rooms), 76 cm (small bathrooms), or 63.5 cm (guest WCs, storage rooms); for apartment doors 101 cm, for front doors preferably also 113.5 cm; windows single-leaf 101 / 113.5 / 126, double-leaf from 151 cm; toilet windows 76 cm.
The corner behind the stairs in child 2’s room is a bit strange, do you have a solution for that?
To build over the first step is already okay like that.
It is 5.5 meters to the garage door, is that enough?
It will be sufficient according to the building regulations and the development plan, and also in practice with a sectional door.
The heating is in the upper floor laundry room so that only one chimney is needed for the stove and heating.
One chimney yes – but not always one flue, that also depends on the fuels.
In child room 3 there should also be kitchen connections so that later a kitchen can possibly be accommodated upstairs and the wall between child 3 and parents removed so that a living/dining area is created on the upper floor. Would this idea be good?
Where do you want to make the division on the ground floor so that a separate apartment could be created upstairs?
Is this plan fundamentally usable or another disaster?
Both. Maybe a little less of a disaster than before. What remains is, in my opinion, your unfortunate approach. Somehow you seem to approach this with a mouse pad horizon. And for the fetish with the most symmetrical front view, harmonious proportions are sacrificed at many points. Aesthetically, that could prevent exactly what you are actually trying to achieve (?)
The minimum furniture size arrangement seems to be slowly loosening up a bit, but the path is still long overall.