How far is the L-block wall from the property boundary to the street? I see the following problem: There is no space in front of the L-blocks for a proper foundation. Even if you ignore the boundary underground, as you yourself write, at the "high end" you need a 4m post. That will also have a corresponding lever effect, and strictly speaking, you would then need an even larger foundation or, possibly, the post might not be thick enough. That’s why my suggestion is to go behind the L-blocks and cast the anchors in concrete. At the "high end" you can simply make them 80cm deep (and theoretically even embed the posts in concrete there). Later on, they can be shallower but longer and wider. Up front, you could maybe even remove the L-blocks again and pour a deep foundation underneath them and then connect everything together with long screw anchors (glued in). That would be the super safe version for paranoids. I would probably just screw the last posts onto shallower concrete foundations (alternatively concrete block steps). That also weighs 100-150kg. If the rest of the fence is properly founded, it will hold because it is connected.