Floor plan design for a single-family house for 4 people on a 390 sqm plot

What is wrong, for example, with the proposed floor plan here with the straight staircase?

I don't see anything wrong with that. But especially with a row house, one could put a bit more thought into the design instead of being inspired by a city villa. For example, the entrance could be placed at the garage corner, recessed with a canopy, and yes: I also see more of a double half-turn staircase here, not because the house is only 10 x 10, but so that both children's rooms face the garden, the bedroom faces east, and a storage room doesn't waste valuable window space.

So far, I haven't come up with anything more useful than the draft from the first post.

Who is responsible for the planning: Do-it-Yourself, but with some floor plans as templates

Why exactly 12? [...] It looks as if the hallway is 2.50 meters wide, which is more than sufficient.

Do you notice anything?
You browse the web and find only the simply designed city villas among 10 x10 floor plans... Planning a house is not really your forte.
The area offers you quite a bit. A straight wall line between the garage and the house is always flat. With a double garage, a storage room of 3 x 3 meters is also possible. The gained 3 meters can be continued on the ground floor and designed as a sheltered terrace with corner windows, for example.
However, I must admit that when sketching, I rather thought of a Bauhaus style ;)