Retaining wall, which variants?

Greywacke ... I didn't know the word before.

I always like to learn something new (and then forget it again ), so I looked it up ...
You are not allowed to post links (not even "for informational purposes"), but a picture is allowed. (this is from Wiki)

A concrete wall that is ground down after casting.

Do you mean exposed aggregate concrete?

Exposed aggregate concrete ... also an option, [ATTACH alt="Waschbetonplatte.jpg" type="full"]46649[/ATTACH] No, I meant terrazzo. ... (doesn't have to be done so elaborately

[ATTACH alt="Terrazzo_retouched.jpg" type="full"]46651[/ATTACH]

have the advantage that you can make them yourself and they are therefore quite inexpensive.

well, those things only cost 2.20, I don't know if making them yourself really pays off. Have you ever done it? Do you have instructions?

I don't think the "doing it yourself" referred to the stones, but rather to the wall itself?

"With the hollow blocks, I also have the fear that the mortar will fall off"

I assume that the durability mostly depends on using the right materials. With a primer and the appropriate plaster, it will last a long time (I say this as a non-bricklayer).
A problem could of course also be the water that can accumulate behind such a wall, but that also depends on the proper construction.
If you have a bricklayer you trust, I would take the risk in your place (if "plastering" appeals to you).

Hollow blocks are also available with different surfaces, do not need to be plastered, and have the advantage that you can make them yourself

I assume that by "hollow blocks" you mean there formwork blocks and then dispense with the concrete filling?