Forget the material.
Forget the feedrate.
Forget the stepover.
I had this same problem quite a few years ago. Fooled around with CV settings until I was blue in the face. Waste of time.
Finally, I quit looking at the cutter and looked at the stepper motor while it was cutting. The stepper motor was turning ever so slightly every time the Z axis changed direction. Imagine that! The four bolts holding it down had loosened up and allowed the motor to turn a little bit. That was an easy fix.
A few months later, the problem came back. But the motor was nice and tight. This time it was the coupler between the motor and the screw. Its little set screw had backed off a tad and was allowing some backlash. The screw would stop at one side or the other of the flat on the output shaft. Fixed that too.
Oh no, here we go again! This time it was the nut on the Z axis screw. It was worn out and allowing backlash.
I think that makes me a veritable expert on this problem, well maybe not, it is quite possible that someday the router itself will get loose in the mount!
Here is the deal. What you are seeing is the failure of the Z axis to lift the tool to the proper height. It is not the Z axis driving the tool too deep. Remember, gravity is helping when the tool is moving down, it is working against motion when the tool is going up.
The real tipoff to what the problem is indicated by some of those very faint lines that slowly disappear as the tool moves away from the edge and Z is moving slowly as the height changes are less severe.
That is my 2 cents worth. If you cannot find anything in the drive train then it is time to mess with the CV settings as Gerry has suggested. There is no point in playing with CV until you are dead certain that you have no play in the Z axis. First things first. My rule for trouble shooting is to always do the mechanical stuff FIRST, before playing with software. It is easier to find mechanical issues than sit around trying to find some magical software setting to cure the ills.
Low Profile CNC Router Vise