How do I create a slope, and how do I best compact it?

Setting a retaining wall and backfilling not an option?

Far too expensive at 42m length and 1.2m height

Look here. Sand, then soil, pressed down with an excavator bucket, smoothed, ready for planting.

Much too expensive at 42m length and 1.2m height

30 meters length and 1.2 meters height and 2500€ material costs

30 meters in length and 1.2 meters in height and 2500€ material costs

If it were only the material. We have had quotes and none under 8k.

We also considered Nordlys planting rings. But our soil is quite swampy below the embankment. We have to work quite a bit with gravel to get a stable foundation. Unfortunately, that also puts us over 5k. And we can't do it ourselves. My old man has back problems (herniated discs) and I am not allowed to lift heavy things in the coming months.

Congratulations! But then you can't do your slope yourselves, can you?