I have also considered L-walls, but such a wall quickly costs around €25,000.
I need to talk to a structural engineer to see what and how I can support it.
Planting stones would also work. Then they should be installed with the appropriate slope. But visually, I think they are the lowest tier. I'll just assume 20m. I think it's probably a bit more. 20m of L-walls in this size cost about €8k. If the landscaping contractor has good conditions, even less. I think €25k is too much. Still, of course, a lot of money. For the formwork stones, you need a foundation so that you reach at least 3m wall height. Mixing yourself is absolutely unthinkable with these quantities, so ready-mix concrete with a pump is needed. I just checked the calculation again for us – that would be about €3,500 for the wall. For the formwork stones, you will spend about another €2,000... price-wise, that's really a whole different level. I hadn't thought it would be that extreme.
Do you need the entire area or not? A 2- or 3-tier solution would also be conceivable. This way, the load is distributed more nicely.