First of all, thanks for your once again very quick and good answers!
Try to add new object, more easier to handle afterwards also and local polycount is big plus always + less shading problems.
@TynkaTopi: You mean adding a separate mesh for the cuboid? But then I would loose the soft joints between the cylinder and the cuboid…
@JA12: I tried to keep the vertex count as low as possible. You’re compensation the problem by increasing the resolution. I was hoping that there is a way to solve the problem without increasing the complexity.
Try creasing the edges instead of adding edge loops.
@m9105826: The creasing gave unwanted effects and it is not possible to precise yet rounded edges along the cuboid.