Hey guys, thanks for your responses and the shared playgrounds.
Because I havent had found any workarounds 4 months ago, I decided to give it a shot by myself.
It surely is no the smartest or fastest approach and its pretty hard to read, even for me now looking back on my code.
My goal was to create a box which has a tiny bit of bevel on the edges and corners, small enough that you hardly notice it, but big enough to add a little touch of realism to the box. I also wanted to use additional geometry as little as possible for performance reasons, so I only used 2 segments and worked my head around the topic smooth shading.
I couldnt use the babylon function forceSharedVertices() to achieve the smooth shading, it was necessary too keep all the vertices (even if they are at the same position) because I had to texture them differently. So I manipulated the vertex normlas by myself and achieved a nice smooth shading effect which looks pretty nice if you dont scale the bevel up toooo much
I did code locally, but the moment I saw some people are interested in this topic I created this playground. Its sometimes formatted a bit odd, sorry for that