So, regarding the kitchen, I notice that the cabinet behind the door is only usable to a limited extent.
I also see the storage room as problematic. You can hardly get anything into the "tube." One idea might be to possibly leave out the tube and have more space for the wardrobe and hallway. Or you could look into whether there are "pull-out cabinets" where you can use the front as a wardrobe and have a cabinet behind it.
Is the slope in the guest room sufficient to legally make it a living space? The question would also be whether you want to better use the hobby room. The largest room in the basement currently has the quality of a storage room, and the second smallest has the largest light shaft.
The children's bathroom seems very cramped to me, as does the guest toilet.
For the guest toilet, you could move it next to the stairs (you don't want to use the window facing the entrance anyway, better ventilation would be preferable), and put the wardrobe without the storage room next to the front door, where there is also more space for putting on shoes.
For the children's bathroom, you could make the bedroom narrower so that the hallway has the width of the stairs, also make the master bathroom accessible from the hallway and reduce its size at the expense of the children's bathroom (then the space for the shower would be missing). The parents' closet wall would remain roughly the same length.
Hardly any light comes into the hallway on the upper floor, and it is very narrow.
One idea would be to install a sliding door or folding door from the kitchen to the living room.
The dining area is quite tight. Do you have to go past it to get to the terrace?