100m² Small House Floor Plan - Too Small?

The built-over staircase will not work. It is probably already relatively steep, and you want to build over 3 - 4 steps there. Example for this (although it is a straight staircase, it shows the problem, and only 2 steps are fully built over)

Yes, that's exactly the problem I already suspected. Especially since, as drawn, 3-4 steps are built over. I have my first consultation appointment next week, let's see what else they come up with.

This can already work. I have something like that. With a height of 1.85 m, no problem. With 3 steps it definitely no longer works.

if you change a lot from the standard, there is always an additional charge, without compensating for not building the standard accordingly. if there are many changes, a general contractor who builds your dream house is already worthwhile. so before you sign, have your house (with all changes already included) offered by other general contractors

Yes.

I thought the OP meant only from 100 m2

"Number of parking spaces: up to 100m² living space 1"

In the original bathroom, the shower somehow seems too small to me; I would have installed a somewhat larger shower. The standard is 90x90, but I would like to go at least 1.1x1.1.

A 90x90cm shower is by no means small, 100x100 is absolutely spacious, bigger would be too much for me; I would definitely want a bathtub in the house, unless adult or younger additions are completely ruled out. If it were just for me alone, I would rather build a "shower temple."
But you’re planning for YOURSELF first and obviously nice showering is important to you. Then just build such a shower that you like and in any case choose a suitable floor plan for it. I read about some rather unclear future options you want to enable, but so far I don't see much implemented from your NOW.

I would also like to put a door between the living room and the stairwell, then the room won't visually seem quite so huge, and you don't always have the feeling that you’re sitting in the stairwell.

Sliding door?
That's probably weird for the "most important" door in the house. And no matter which wall I let it slide on, it restricts the use of the room.

Sliding doors are mostly open in my case anyway; in addition, I mostly perceive them as a makeshift solution and inconvenient to open. Currently, we have a wide glass door between the hallway and living room... it is always or 98% open. I don’t take heating issues that seriously, plus I would find it better if you don’t walk into a cold area when you want to go upstairs. Through conscious design and decoration, you can also make the stairwell appear as part of the living space; maybe you’ve so far imagined rather cold stairwells; you definitely won’t perceive it as huge. Maybe it could also be a nice curtain between the antechamber and hallway, i.e., further forward in the house, which you use only in winter, for example.

I think that at mid-30s, I am far enough in life to say this not just from youthful naivety. I have also never been the child person who wanted to hold and cuddle every baby...

Well then, that’s a position and I would orient my house project exactly around that. If things turn out differently than planned, you can’t foresee now whether a potential partner might bring three children of their own, four dogs, or live in Timbuktu. Ergo, I would plan for myself alone but also so that a partner who likes my lifestyle can find space in it; if not, it will be sold/rented and something new will be sought, of which you will then also know where and what it should be.
Currently, it seems to me more like the egg-laying wool-milk-sow, i.e., the lazy compromise that is supposed to enable everything but perhaps pays too little attention to your very own interests. Have you looked at houses & floor plans from EBK-Haus? I really think they are great. From what I read so far, I could rather imagine a bungalow with some additional space upstairs as a gallery like, for example, has. Currently, you would have three closed rooms upstairs, mostly unheated, and everywhere something is standing until maybe someone comes who wants to use it.

The original floor plan is clearly more usable on the ground floor. You could provide a sliding door (running inside the wall) to the kitchen, and also to the utility room.