Is it possible to make a displacement texture where each cell of a voronoi texture results in a 100% flat planar surface?
I have adapted my first approach a bit and also fed the Normal slot of the Glossy Shader to create the impression of a perfectly flat surface. If you want to also achieve the beveled look in your image, I am hitting the boundary of my knowledge/trickery. Can't help you with that. But this way at least the convex surfaces disappear.
I don't know your exact use-case, but I believe Geometry Node might be the better way to approach this.
I replaced your bump map with a proper displacement map, fed it into the displacement slot of the material output and played around with the exponent value of the math node as well as different Voronoi settings.