Should the land be filled up or not?

That depends on what you want to build and where the building is to be placed. Do you want the "certain flair" or maximum return? Here is maximum return: House dimensions approx. 11.80*11.50, ground floor, first floor, attic as non-full floor, 3 residential units, 3 parking spaces House entrance accessed via a path from the parking spaces, thus a longer route to overcome the height difference. For true accessibility, the finished floor top edge would have to be about 50cm lower, i.e. 22.00, or your planner comes up with something better, e.g. parking spaces in the west or with centimeter optimization considering the floor area ratio further to the south. Edit: Oh! Missed that. Show me what you are planning (size, placement)

A single-family house with approx. 140-150 sqm is to be built. The currently attached plan is unfortunately no longer quite up to date. The house shall be deeper and therefore less wide so that we can have "more" garden in the west, which is not exactly easy with the plot size. The terrace should also be in the west.

Great how you created the cost-optimized variant. That is once again extremely valuable for the OP.
The term "maximum return" is somewhat misleading – one form of return in house construction is the satisfaction that arises from living in it. This can definitely be "flair-determined." As a builder, never forget in all priority settings regarding the wallet that the purpose of the house is habitation and not "saving."

That is so valuable for us, that's true!!

For that, I am also very, very grateful to the participants here!

Enjoy the three-dimensional conditions! Some variation in the elevation profile of the plot definitely adds a certain flair. Ask an architect how they can play with the conditions.
Ours had an incredible imagination for placing the now built house for our life on a very three-dimensional plot.
By the way, the top edge of the finished floor is 10.5m above street level.

The "problem" is that we haven't found anyone yet who you felt was really into it... We've had 3 conversations so far and in each the first comment was "the plot isn’t particularly big"... As if we didn't already know that.

Does the garage/parking space have to be at the same height as the house? Otherwise, you can consider them separately. Parking space at street level, house at a different one.

Does the garage/parking space have to be at the same height as the house? Otherwise, you can consider them separately. Parking space at street level, house at a different level.

Honestly, I’ve never really thought about what it would be like if you looked at the parking space differently. But it’s not a must that it’s at the same height. We would even prefer the driveway to be as gentle as possible!

The "problem" is that we haven't found anyone yet where you felt like they were really into it.. We've had 3 conversations so far and in all of them the first comment was "the plot isn't particularly big"... As if we didn't already know that.

YES, that is really the biggest challenge. We also searched for a long time and then got lucky. If you were in Oberbergisches, I would have a tip for you. You can generate interest by sending a few emails to architects in the region: "We are looking for an architect who will build exactly the right home for our life on a small plot with a slope away from the street. We know what kind of living feeling we want and cannot translate that into architecture ourselves. The budget is xxx. How interested are you in this challenge?" Then the individual part: "On your website, we were attracted by this and that" or "In an article, we read about you..., that appealed to us" or "You were recommended to us by xy" Principle: No flirt without personal approach. Feel free to also knock on doors of people with appealing, rather new houses: "Who built this for you?" It will work out!