Floor plan single-family house approx. 180 sqm

A straight staircase is simply not an easy structure, as many things must be taken into account or the planner must be skilled. The shorter the staircase, the steeper it becomes. Of course, this also depends on the floor heights. If you don’t want a "Hühnerleiter," then a straight staircase with normal floor heights will be about 4-4.5m long.

We are currently working on our floor plan and are slowly getting desperate.
[...] Maybe someone here has a good idea on how to still save the whole thing.

Nothing easier than that: go to an architect, who does this professionally (I mean building planning, not "despairing").

We have now had the floor plan created that is optimal for us.
We are super satisfied with everything except for the fact that the Child II room is cut too narrow for us.

So without the kinked second children's room, the drawn plan would have seemed ready for construction to you—apart from the total staircase length that took you by surprise???
Then there is still a long way to go. You have already been given the key several times.

If you order a "floor plan 2 go, 180 sqm with straight staircase" from a general contractor, you get exactly that. What you are showing here is an aesthetic drawing (from a layman’s perspective, “professional”) of pretty much the biggest nonsense I have seen in a long time (including the green forum, where I have been for six years and two days). What I “like best” is the floor-to-ceiling window in the utility room.
Was there any show house as a template, or how did the misery start?

Draftspersons have not learned design techniques in their training, so they cannot really help the client in this respect. Then nonsense like a king-size guest WC with a door that still has to open outward, a randomly structured upper floor with a 32 sqm dormitory, and overall 180 sqm of floor space to be cleaned but without offering anywhere more than a 140 sqm "small" house arises. Tell me, what were your specifications and where do they come from? Draftspersons "do not think," so the seed for an unconvincing result must have been planted by the building clients.

P.S.: Not the draft itself, but the assumed approach reminds me of the thread by (there based on a Bien-Zenker M165 Wuppertal) – however, the draft shown by you rather seems to me as if you wanted to rebuild a Hanse varied with Heinz von Heiden.

Are there no plans of the property and/or pictures of the surroundings?

Stairs with a 26 cm tread are quite borderline. 28 cm are much easier to walk on.

How high is the ground floor?

What kind of strange wall construction is that?

Here is a suggestion from the internet. You can adjust it.

Is it possible to build the draft at all without additional columns in the living room?