Your work on fixing the issues with the scanlines is hugely appreciated, thank you
If I download your screenshot and zoom into the .wip section then the scanlines are still not even despite being calculated from a pure white background of the same colour.
It looks like the brightness of each scanline is being calculated by a sinusoidal equation of some sort but the offset of the peak brightness is being shifted.
The following is taken from your .wip picture:
The left hand side shows the scalines at the top of the image, the right hand side shows the scanlines slightly below the scanlines shown on the left.
In this case the thickness of each scanline is even. The brightness is not.