Botch on the interior staircase or do I have to accept it like this?

On what basis should acceptance be refused here? There is no planning; that was left to the general contractor or the staircase builder, presumably the master stair builder(?).
If the customer does not give me any specifications, I design the HMI freestyle, as I think it fits.

Certainly, a few minor things need to be corrected. But you cannot reject a staircase worth 10,000 EUR for optical defects costing 500 EUR and not pay anything.
[...]

Thank you for the different perspective.

What does "HMI" stand for?
Which points do you think need to be corrected?
Is "no planning" or "on-site planning" by a master stair builder not unworthy, or is that also your own work standard?
Would you accept such a staircase in your own home?

Nerves - exhausted.
Money - exhausted.
Humor - exhausted.

Unfortunately, there is little room for maneuver when there are signed contracts. It is clear that the initial mistake was signing them in the first place.

However, it doesn't help to dwell on the past, but to look at which active rescue options, arguments, and leverage are available to me here in order to get the best possible outcome.

What can I bring forward here?
Thank you

As I said. When the child has fallen into the well, it's hard to change anything. If you never saw the planning, it just somehow went poorly. The cutouts for the stairs are probably quite complicated as well. At least for the stairbuilder.

So my stair builder (however commissioned separately) sent me a technical drawing (with technical dimensions) and a 3D visualization after the sample selection. The staircase was only put into production after my approval (by email). So what is really a defect is the installation height of the short parapet. In my opinion, the finished floor height should be taken into account there. Everything else is not nice, but in my view not a defect.

There was no such thing as "planning" with us. The process was: sampling, additional agreement, installation.

The small parapet part is not yet screwed on so that we can tile underneath. The stair builder had demanded from us that we do not lay the last row of tiles yet so that there are no problems with the stairwell cladding. Now, however, I am asking myself how one is supposed to tile below the start of the stairs from ground floor to upper floor and also where the (non-sampled) posts are. To me, it now seems that he talked us into something that makes the work easier for him but significantly increases the effort for all other follow-up work, which I did not understand at all. It could have all been tiled two months ago...

Well, if nothing was specified, you can't really report more than possibly existing craftsmanship defects. If you don't like the look, you should have made sure during the planning that you know what you're getting. Whoever orders "one staircase" will only get "one staircase." Since in construction you mostly deal with people who think no further than their own nose tip, you really have to watch out for everything yourself.

I find the staircase generally quite ugly. Did you even want such a construction or did you only specify the type of wood?