Ideas for slope landscaping, garden house, ...

It looks as if all 4 sides are sloping downwards?

Stable foundations can be made everywhere. It is always a question of effort.

Here, it would seem most appropriate to me to plant the inclined surfaces with something that stabilizes the slope and grows just as high as is right for you.

Hey, have you looked at the picture?

Yes, but it hasn't helped yet. Show it again with more surroundings in the picture. I wanted to understand better how the viewpoints / directions of the other pictures are, to piece them together in my mind into an overview. It wasn't enough for that yet (and at first I thought it was only me). But now hardly any comments have come from others, so probably more readers are also clueless about how it all fits together.

that looks like it’s going down on all 4 sides?

Stable foundations can be made everywhere. It is always a question of effort.

Here it would seem most appropriate to me to plant the sloped surfaces with something that stabilizes the embankment and is just as high as is right for you.

Yes, we are considering that too.
It goes down on all 3 sides, it is a hillside property. On the left towards the playground we might build a wall, but first groundcover and bushes below. On the right is the neighbor’s wall.

Why a wall to the playground?
Plant the embankment with ground cover, also something a bit taller
I would wait with a garden house. You have filled in quite a lot. Who knows how and where everything will settle.

It slopes down on all 3 sides, it is a hillside property.

... the main area of the property looks flat (to me ... in the photos). I would rather describe it as a plateau property ... and the question is, what do you do with the slopes around it? Of course, you gain some space if you fill up to the edge, but that also costs money and good planning to ensure a retaining wall holds. If the space is not foreseeable to be needed for anything, I would plant plants. Advantages: You can determine the height and (within limits) change it later. ... Plants are somehow also greener than walls ... ;-)

Why a wall to the playground? Plant the slope with ground cover, also something taller I would wait with a garden house. You have filled up quite a lot. Who knows how and where it will settle.

Yes, you are absolutely right about the settling of the soil, we already have quite a hole where the house is actually supposed to go. We are waiting a bit longer.