Barrier-free single-family house floor plan for aging with parents' bedroom on the ground floor

Unfortunately, I don't have any measurements; I need to work on that with the architect. Does it perhaps make sense to banish the technology room to the extension?

and the bathroom past the hallway by the front door?

Why does the bedroom have to be somehow on the ground floor?
It is only conditionally suitable for limited mobility. If the stairs are a problem, there are stairlifts

Because I find it very pleasant when the children have their own floor. We now live in a maisonette apartment and also have the parents' bedroom on the ground floor there. This is a very personal preference that I definitely do not want to miss.

Why people place a sofa with the backrest against a garden window and install a screen as the central element in the living area will probably remain a mystery to me forever.

You are a sound specialist after all. There are people, like us, who sometimes have a proper home cinema evening. In principle, you will surely agree that for such a target group, your criticized variant is still better than a TV in front of a large window, including center and front speakers. Then you might as well do without the window. From a pragmatic point of view, a window should bring light. It still does so with such a setup.
When I watch TV and enjoy the sound, I don’t want to look out the window. I don’t do that in a cinema either.
That said, when we simply sit in the living area, we always sit at the dining table, not on the couch by the window. There we have more or less a panoramic view outside.
Conclusion: a place should be designed according to daily habits. One person sits on the couch and wants to look at the greenery (if at all possible), another sits on the couch and has a cinema/TV evening.
All correct from my point of view.

All this especially against the background that few people have the space and above all the money for their own cinema room.
If you like to watch TV, that is.

There are people, just like us, who occasionally have a proper home cinema evening.

a place should be designed according to everyday habits.

Absolutely right - it serves everyday habits. If film art and TV are important, there are solutions without the compromise of sitting with your back to the garden, for example a hidden motorized screen for the cinema. That also works with the sofa facing the window. Speakers can be planned In-Wall in a new building; there are excellent systems for that. For casual watching, a small, subordinate screen is sufficient. In so many other areas, thousands are spent to have better hinges in the kitchen or a Q3 wall finish, or lots of built-in ceiling lights Q1–Q4 or generous traffic routes, or... I often find the creativity lacking there. If the solution sofa facing the window has been weighed up, that's fine. But I believe it is rarely really thought through.

Because I find it very pleasant when the children have their own floor. We now live in a maisonette apartment and also have the parents' bedroom on the ground floor there. This is a very personal preference that I definitely do not want to miss.

That is something different than barrier-free in old age. Don't plan a parent wing as a loveless appendage next to the entrance. If only because you hear everyone coming and going.