Single-family house approx. 180 m² - What do you think about that?

First of all: congratulations on the successful course of the design. Certainly, one or the other thing can and should still be adjusted, but it is a mature design.

The 17m² is large, do you think the bathroom can be furnished differently? The decision to have only a shower on the ground floor was made consciously.

17 sqm is very large... minus 4 sqm sauna leaves 13 sqm. Still very large, considering that on 6.5 sqm you can comfortably accommodate a bathtub and shower with double sink and toilet. This here is a dance hall and has that kind of coziness too.
Under the stairs, you could make a closet niche with shelves for bath towels. It would be better to accommodate a niche for a separate toilet there. In the corner, behind it, where the sauna is now, a spacious shower temple can fit.
There will definitely be space for the rectangular sauna.

Regarding the sauna, we were told that 2 x 1.9-meter external dimensions are sufficient. Do you have any experience here?

I’m wondering: do you have no experience in this regard?
And shouldn’t there be a plunge pool, i.e. a tub, where the sauna is?

BUT this corner is still the first thing that catches your eye and jumps at you

No...

Indentation" that ruins the sense of space for me.

No...

The corner is okay. I would also make a door between the living room and the hallway. And move a few walls, for example give the sofa a bit more space, the bedroom a bit less.

Entrance in the north fits well too.

P.s. The end of the stairs is a construction error... a wall needs to be built along there. Also, the stairs will have to get a landing. They are currently hanging open.

Furthermore, I would keep the living room bay window and straighten the exterior wall on the south side. The stairs are actually a mess there.
The Katja corner would benefit if it is aligned with the load-bearing wall.
The toilet would benefit if another one is not added, if it can only be reached by crossing the hallway (illness, at night, stairs).
Straightening things out would probably open up entirely different possibilities.
But I think you could go to the architect with this. He will fix the defects that are not so obvious to a layperson now.

Do I understand this correctly. The house has a huge roof terrace and in the south a balcony strip of about 1.5m? This means that in the south the concrete slab protrudes from the house. How do you insulate or build something like this properly? I don't want to speak badly of it, I am just interested.

No...
No...

The corner is okay.

No...
You come in and automatically turn slightly to the left into the open space, and your view and walking direction go straight to that corner. I think if the "Entree" of the living area were different, it wouldn’t bother me so much. But yes, well, if everyone finds it okay...

Nope...
You come in and automatically turn slightly to the left into the open space and the view and walking direction go straight to that corner. I think if the "Entree" of the main room were different, it wouldn’t bother me so much. But yes, well, if everyone thinks it’s okay...

As I already said: certainly a challenge from a construction point of view
I would straighten every wall anyway.

Well then:

Oh, that really is a corner where yellow-black striped bump guards belong.

As I said, in an apartment building my flat could look like that and it would be okay. But in my house, that is a "recess" that ruins my sense of space.

No, I wouldn't like that even in a flat.