Thickening an L-stone using formwork and concrete?

yes, I am still thinking about the curb

The problem is that the L-stone is cast in exposed concrete quality. Therefore, it has a very smooth outer surface. The newly poured concrete cannot adhere to it (even with reinforcement), and moisture will always penetrate the gaps, causing the space to open up. In addition, it looks really bad because you will have different concrete appearances (color, texture). I would form a shuttering and pour concrete on top, and then possibly put the sheet metal over it (it will only get very hot when the sun shines on it) or maybe work with cladding (there might also be ones in the same color and texture as your terrace slabs).

yes that's how I suggested it
I don't think I understand you right now
first you say the newly poured concrete won't hold and will crack, then you suggest formwork
I see a contradiction in that.
Or do you mean only the metal lid prevents this because no more water can get in?

Why don’t you make an "Attika" over both and then pave it there? You simply backfill the area of 12cm at the l stone. That’s all there is to it.

On the wall, a railing or privacy screen may still be added if necessary. To what extent it will ultimately be a metal sheet or something else, we will see then. The parapet would have the advantage of rainwater drainage and protection of the supporting wall from drips. Therefore, the primary question is how I can add an additional layer in a meaningful and durable way.

therefore you don't have to add extra for anything or anyone, so what advantage does that bring? Then I would put a "Granitfensterbank" on it, there you can also provide fixtures for railings & co