Is the excavation suitable for backfilling?

As long as it is not used as a substructure for the terrace, there is absolutely nothing against it.

that's how it is, suitable purely for profiling but please not under the terrace. As already mentioned here, it will settle even after years and you will always regret it. Clay-rich soil is like dough and cannot be compacted.

Just a quick clarification:

The terrace is to be built below the balcony (see picture Post #6). So the balcony is basically the roof of the terrace. The entire surrounding area needs to be filled up by 70cm. Reference points are, for example, the protruding gray inspection shaft or the green insulation below the basement. Is the ground suitable for this?

Why do you have to fill it up? Just make the terrace level with the house and then fill in a bit with fresh humus for the lawn. Then you basically have a veranda that is slightly higher than the lawn. I don't find that bad at all!

I would rather terrace your slope with the excavation and make it usable.

That would actually also be a possibility, however the inspection shaft would still protrude. It was planned at the prefabricated height back then. Is there a suitable plowing device or something similar for the slope to eliminate the craters?

Milling and straightening.
Plough goes too deep and becomes very rough.
Must be suitable for the slope inclination or be so light that you can still control and manage it.