Uncertainties regarding size, planning itself is so far completed

I notice that the middle third of this area is completely empty space. I would not say that the area is too small, so perhaps the arrangement of the furniture should be reconsidered.

Have you already thought about how the kitchen should look later on? As on the plan above, there is only a small kitchen unit without any storage options. There is also no storage room on the ground floor, which means you would have to store most things in the basement. That’s not a problem, we also have no storage room planned on the ground floor.
The empty space in the middle of the living/dining room looks wasted. If you give up the corner bench, you could really arrange it nicely:
Along the top of the plan on the outer wall a kitchen unit, below that a cooking island that connects to the left wall. The dining table in the middle of the room and the couch at the bottom, just as it is already drawn in. The living room door would have to be moved slightly downwards on the plan. I attached how I imagine it.

Our floor plan is similar on the ground floor, but with a separate kitchen; instead, we planned an office in the basement, have a look at it:

P.S.: Our house will also be only 10 x 8.5 meters.

4.25*9.13 = 39 sqm - not huge but okay. The zoning is very difficult like this - such a very open room with the 3 functions poses a certain challenge to the design of the furnishings - possibly drywall? That would be too difficult for me - either get help or consider going around the corner. Swap the room with the kitchen. The room would then be somewhat narrower but fine; a U-shaped kitchen would fit well in terms of width and would be somewhat separated from the living area. Access to the room via the corridor, to the kitchen as well.

Somehow in that direction:

We are not subject to any building regulations regarding the number of floors, we just don't want a house that is too tall and no flat-pitch Tuscan roof, hence the raised wall

For your unusual request, you are making a huge compromise. These slanted walls are only made because in many development plans it is not possible otherwise due to building regulations. If you are allowed to build 2 full floors, then just do that.

You are now at a total height of 8.40m? With 2 full floors and a 25-degree roof pitch, you hardly get higher, but you can use the rooms in the upper floor MUCH better.

With a knee wall of 150cm, one can hardly speak of idiosyncrasy. While in a two-story building straightforward rooms with space for furniture score points, the sloping roof can create a cozy atmosphere with an acceptable knee wall height. However, I see problems in the statically predetermined window arrangement on the ground floor, as hardly any window is intentionally placed there and it causes more problems when arranging furniture rather than being sensibly arranged.