Improve floor plan - how?!

You indicated a slope. How much within the building envelope?

Shutters definitely for shading purposes
Inform yourself about the difference between air-to-air heat pump and air-to-water heat pump

I don’t like the upper floor either. In addition to the points you mentioned, there is a children’s room on the dark north side and the parents’ bedroom (without shutters :eek) on the south side. I would do that differently in the next draft as well.

Bedroom in the south is a pity, since you only sleep there anyway. Children's room, on the other hand, in the darker northern room. South-facing without roller shutters will be torture; in summer you'll be sleeping in a sauna. saw it correctly: instead of giving a city villa a knee wall, it would be better to work with a different house type here (which I call a 1.5-story building) as a basis.

You indicated a slope. How much is in the building window? Roller shutters definitely for shading. Inform yourself about the difference between air-air heat pump and air-water heat pump.

The plot has a width of 20m and a depth of 25m. It is approximately level for the first 12 meters and then slopes down by 1.75m towards the property boundary. So a kind of terracing towards the south garden. Do you consider the air-air heat pump unfavorable?

Do you consider the air-to-air heat pump unfavorable?

Don't discuss technical matters in floor plan debates. Don't mix everything up in your head.
I also believe: wrong initial design.

You need a different starting design (with a pitched attic, with some providers this is called "country house" - don't worry, the Forsthaus Falkenau is not meant, the style can be "urban-modern"). Usually, you are best off with "rectangular" rather than square floor plans.

Admittedly, I am a bit overwhelmed and don't know offhand how to solve it.

First, the house, the design, then everything else in the discussion.
Read up here. Unfortunately, we do not know your ideas regarding the plot and usage. But take a look and draw as instructed in the advice thread. Just try sketching with rectangles or grids. Then look for a similar floor plan that already exists as a gable-roof house. Have it calculated and redrawn by your draftsman as 70, 55, and 40 (energy standards) and then have it discussed here. 40 is a gamble like playing the lottery or buying stocks - you have to be ready to lose the money. Better treat yourself to reasonable windows of at least 2 meters and smart kitchen planning with the money.

Don't discuss technical matters in floor plan debates. Don't jumble everything up in your head.
I also believe: wrong original design.

This way of responding confuses me.
I didn't even bring up the heat pump topic...?