What do you think of the floor plan? Do you have any suggestions?

YPG, why shouldn't the kitchen be where it is? We've done something similar in our place. A 4-gable house and in one gable we have the kitchen and the living-dining area is also on the side across the entire width of the house. Only the stairs and the other rooms we've arranged differently.

Exilhamburger:
I don't like this hallway upstairs. Long, narrow and dark. Unfortunately, many still think that a hallway is wasted space. But that's not the case. In our hallway upstairs we have room for a wardrobe, a shoe cabinet and a mirror. I would leave out the small hobby room and just have a big hallway here or only separate it from the hallway by a half-height partition wall. That would make it look much more open.
The dressing room will be quite dark, you will need artificial light here during the day to see something.
For child 1 you don't have a good spot for the wardrobe. Either you run straight into the wardrobe when you open the door or you block the window. As nice as big windows are, you should always plan windows and doors with a distance of 65 cm from the wall so that there is still space for furniture.
Do you have a basement? If not, the utility room will be quite small. If that’s where the technical equipment and washing machine are, there won’t be much space left for anything else.
And I also see the problem in your living room that you always have to walk around the table if you want to get to the terrace. I find that very impractical, especially with children or if you have celebrations.
For your guest toilet, I would move the door to the other side of the wall. That way there would be a nice niche for a cupboard or a coat rack on the right hand side of the hallway wall, and in the guest bathroom you would also have the option to put a cupboard or some hooks behind the door for towels and bathrobes.

The hobby room is actually a study. My wife can do home office on the condition that the room is at least 7sqm. Therefore, it has to stay. What do you mean by mirroring the bathroom on the ground floor? How else should the upper floor be divided? I had also thought about placing the table directly at the kitchen island.

YPG, why should the kitchen not be where it is? We solved it similarly in our home. A 4-gable house and in one gable we have the kitchen, and the living-dining area is also sideways across the entire width of the house. Only the stairs and the other rooms we arranged differently.

On the upper floor or ground floor? The walls are made of glass

Rain retention basin