Designing a garden on a slope sensibly

Our highest point is 225.70. The original plan was to use that as the starting height and fill the rest of the garden with soil. However, we have now discarded that idea again.

So you would basically fill up to the width of the house and terrace down towards the parents?

Yes, maybe step down towards the parents and then build a higher wall at the front towards the street at the parents' boundary.

Just as an example, the left side is about 50 cm higher than the right part. This gives our play/dining area 2 levels.
Works quite well.
On the left are the trampoline and the balance beam, on the right swing, slide, sandbox.

That's not a bad idea! We need to take a look at how it would then look on our small property. I just took a few recent pictures, where you can see the slope quite well. It would probably make sense to level the red-shaded area and terrace the rest, right?

Is the white one your house or your parents'? Is the mound of earth still on your property?

is the white one your house or your parents'?
Is the earth mound still on your property?

The house is my parents'. So far, we only have a construction pit. The earth mounds are still mostly on our property.

Good, then the excavation is there. I'll think about what I would do - I'll leave the budget aside and what your B'Plan allows. You have to check that.