Thickening an L-stone using formwork and concrete?

So you would leave the "indentation" and pave "deeper" in that spot? Since it would be visible from the side, and we sit right there, I would find that less visually appealing, but of course it would be the easiest way. On the other hand, we are planning a differently colored border of the paving, so that would be visually even more "dramatic".

I would gravel up to the same height as the paving, then either put a parapet above or a windowsill --> always the same width so that it runs in a straight line, the advantage of the sheet metal would be that you can let the bending, for example, end 1 cm above the finished floor surface. Then, in my opinion, you don't see the wall recess at all anymore.

yes, possible. But I want to avoid a cavity here.

On the other hand, we are planning a differently colored border for the pavement, which would therefore be visually more "dramatic".

Or the opposite: the color change simply doesn't accommodate the offset, and visually, calm is restored again.