Use L-stones for slope stabilization.

: About €10,000. However, this also includes the demolition and disposal of the old retaining wall.

: Thank you very much. I will pass the compliment on to our civil engineer/landscape gardener. We are celebrating, among other things, an opening party on Saturday.

In our new development area, I have now often seen that instead of L-shaped stones, people build a (nice) dry stone wall, and then pour the actual support behind it themselves. The result is, for example, a nice Boss Intakt wall that you can build by hand, and behind it you have to calculate how many cubic meters of concrete and steel you need. For 30m and a height of 1m, if I haven't miscalculated, I come to about 10m³ of concrete. In other words, even if you have it delivered, the concrete plus steel costs a maximum of 2000.-€, gravel another 500.-€, and a bit of consumables plus of course the concrete stones, which in my opinion are much nicer...

In our new housing development, I have often seen that instead of L-shaped stones, people build a (nice) dry stone wall and then pour the actual support behind it themselves. The result is, for example, a nice Boss Intakt wall that you can build yourself by hand, and behind it, you just have to calculate how many cubic meters of concrete and steel you need. For 30m and a height of 1m, if I haven't miscalculated, I come to about 10m³ of concrete. In other words, even if you have it delivered, the concrete plus steel costs a maximum of 2000.-€, gravel another 500.-€, and a bit of consumables plus, of course, the concrete blocks, which in my opinion are much nicer...

A nice dry stone wall can effectively disguise the ugly concrete surface. However, it makes sense to continue building the actual support behind it with L-shaped stones. The purpose of the L-shaped stones is that the weight of the slope to be supported rests on the foot of the L-stone, which provides stability to the securing stone (in addition to its own weight). At least for greater heights (e.g., 1.50 m), otherwise the earth pressure on a placed (poured) wall could be too high. Our neighbor's wall (self-built and filled with steel and concrete) has already tilted somewhat over the last 18 months.

I meant that you cast the concealed concrete wall like quasi L-blocks. Insert a steel mesh at the bottom in the footing, a little more metal in the "bend," and a few more braces going upwards (preferably bent over the footing into the "head"), don't skimp on the concrete, and cast the L-blocks yourself in pieces of about 2m. Up to 1 or 1.2m height is still quite feasible. If about 1.2m is needed, maybe 0.9-1m of concrete is enough, the rest can then be caught by the dry stone wall.
Of course, add retarders to the concrete so the footing doesn't harden too early and then work quickly with 2-3 people and a concrete mixer, then it should be no problem. If you mix yourself, the price per m³ of reinforced concrete (i.e., including steel) is between €100 and €120, meaning including gravel you are still well below €2000.
Depending on soil conditions, you have to see whether you dig the pit yourself by hard manual labor or order an excavator. With normal soil conditions, you should get by with about €500 for digging and transport (at about €20/m³). That would put you a bit above €2000. However, then you need to add the stones for the dry stone wall, at about 30m² that can already be around €5000, depending on what you want and how cheaply you can get them.
For max. €7500, you then have a nice wall and not just a bare concrete surface...