Hi Alex,
There are three things to consider in this set up:
• The phong angle as you mentioned. Select all the Edges and go to Mesh> Normals> Break Phong Shading. The Use Edge Break in the Phong tag needs to be checked for that.
• There are some N-gons in it, and I’m not certain how you like to have these handled. I would pull them out. Mesh>N-gons>Retriangulate N-gons. Same for: Mesh> Commands> Triangulations. Do this before Edge break!
• Lights, well soft shadow is the worse idea to work with, as it is not a real shadow. It is a left over from the early days, and creates dots in space. From there it is evaluated if an object is in shadow or not, but it is only an interpolation, nothing that I would ver use in a professional set up. It is a fake. Hence you see stripes on the surfaces. Yes, I know, other shadow option take longer to render…
As a side note, the Lights> Details> Contrast is on default: Check 30% and -30% out, and go from there. The geometry “frame” seems to create some trouble as well.
All the best