Is staircase DIN 18065 mandatory or not?

We just want to save 30-40cm so we don't have to redesign everything again.

May I ask how old you are? Until the end of my 30s, I also thought that stairs only become a problem when you are over 70. I am now in my mid-forties and notice that stairs are less enjoyable. My wife has had trouble for longer due to torn cruciate ligaments. On the one hand, it’s because of the eyes, which after 25 years of monitor exposure develop funny quirks, so that depth perception – especially in non-laboratory lighting – no longer works well. On the other hand, the muscles, despite or because of training and slow regeneration, don’t want to perform like before anymore. After an 80 km bike tour, I walk the next day as if I had done a Tour de France mountain stage. The last thing I would build into my house is a "oh, it will be fine" staircase – neither for design nor cost reasons. The DIN standard actually makes sense because it is based on experience and ergonomics.

A staircase is not a wall that can be altered if necessary, nor a door that can be widened if needed.
I would never impose a compromise or an "overall, it should be fine" on such an important part of a house.
I even go so far as to say that if a "proper" stair size does not fit into a house or a floor plan, then it must be redesigned until it fits. And: Not everything that somehow fits or goes unchallenged should really be implemented.
I recall a post in the middle of the access that was planned, and the planner initially told us that one could easily walk past it. From today's perspective (without this post)

And no matter how long it would take; I definitely would not want to have this perceived deficiency with me possibly for my whole life afterwards.
My request would not be, besides posting the house project, to once address all the hints mentioned here directly. I read interesting things while going through it, each of which would deserve a concrete answer.

Which interesting contributions did I not respond to?

Until my late 30s, I also thought that stairs only cause problems when you are over 70.

I remember a post that was planned right in the middle of the entrance and the planner initially told us that one could easily walk past it. From today's perspective (without this post)