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Your results are amazing midzki, but there's a few ambiguities that need to be clarified.
For example, to which layer should the filter be applied, from which layer should a duplicate be created, the sequence of layers, and from which layer should the mask be created, and to which layer should it be applied. Although someone who understands the principle lying beneath your processing is not likely to mess up, it would still help if the processing is made clear.

For example, consider there is a white scratch on a black background, if you create a mask from the unfiltered background, the white scratch will be retained because it's white and is masked from filtering. Creating from the filtered layer avoids this problem.

Also, about the relationship between noise level and brightness, I think it's not always the case with screening. An example is this image:



The darkest areas contain much less noise than the highlight areas in this image. Your assumption is quite reasonable because black ink can easily create a large contrast with other color inks and white paper, introducing high luma channel noise level. But if an area is dark enough, the noise level begins to decline with darkness level on the contrary. I think grayscale masking is a powerful method for preserving bright areas from overfiltering, but users should be aware that this masking method also has some deviation from real noise levels and should limit masking strength and look out for overfiltering in dark areas.