Our dream of a single-family house - feedback welcome

Exactly what you say.
The projections are in there. They probably cost more than if you run them straight up. They have to be sealed accordingly.
A simple architecture is "square, practical, good".

wobei 170qm "auf dem Dorf" jetzt kein riesiges Bauunternehmen ist

It's like what is so demonized among craftsmen: "we've always done it this way."
Why not just really think about what you need.

What you have as living, kitchen, dining, and entrance is what entire families live on in Frankfurt.

Regarding the costs:

And exactly, our naive estimate amounts to about 350 including incidental costs and garage, whereby the latter is to be seen as a bonus - depending on how and where we could save elsewhere [emoji6]

We don’t live that far from you. Limburg is also about 30 minutes away from us. However, we probably live in the other direction, towards Westerwald ;).

We are currently building 160 m² + double garage (6 x 9 m). Without basement. Our plot has only a minimal slope (about 1.20 m height difference over 21 m plot width). For the house + garage + incidental costs + outdoor facilities (only the essentials, 15,000 € of which is planned as own work) we are at almost exactly 300,000 €. Without land, kitchen, etc... Rough equipment of the house: 2 full floors, solid construction, square building shape without projections forward or back, air-to-water heat pump, controlled residential ventilation, colored RC2 windows, electric shutters everywhere and partly venetian blinds, sanitary and electrical equipment slightly above standard but no KNX or other gimmicks, no fireplace. Otherwise "normal". We contribute moderate own work (drywall, painting, flooring and such). We are building with an architect and individual awarding of contracts, which in our case is cheaper than building with a general contractor.

For you, I don’t consider 350,000 € including basement, garage, and incidental costs realistic. It might possibly work if you are really disciplined, contribute a lot of own work, and choose inexpensive equipment. Although I would suspect that even then it won’t work. I would almost certainly suspect that you would have to increase your budget quite a bit if you want to build your house like this. Your earthworks will probably swallow quite some money because of the slope, as will the outdoor facilities. The basement really hits financially. And timber frame construction is not exactly the cheapest way to build either. Additionally, you have 170 m², which is not exactly small.

Otherwise, I find the floor plan quite nice. To me it would be a bit too open, from the hallway through the kitchen to the living room, the whole area. But that is a matter of taste.

The table measures 1.35m*2.31m

But that's really a simplistic calculation. If you measure it proportionally, the table is just 170 cm long. And the staircase is roughly 345 to 350 cm long (on the ground floor), which I would consider the absolute minimum. And upstairs (measured out) it's about 365 to 370 cm long ;). To bring the garage downhill and the entrance door closer together, an entrance door on the east side would also be conceivable, it just changes the room layout a bit.

Is a basement really desired, aside from the slope? You could build two stories including the basement: living area downstairs with access to the garden and the bedrooms on the ground floor. So without an upper floor. This keeps the construction costs lower. By the way: where would your terrace be? Regarding accessibility: position the door from the office to the front area near the WC so that you don’t always have to walk around the stairs later. An island is a small island: with a cooktop island, it should be at least 180 cm long. About the dining table: my chairs stand about 30 cm away from the table, so it looks a bit cramped with you.