I integrated it, though the shader can still be cleaned up, now I don’t have to go into the GIMP and overlay the shadows layer.
Left: “ink” shadows are fully black
Middle: “ink” shadows are mixed
Right: No “ink” shadow layer
The material also needs to have the Blend Mode and Shadow Mode set to something that supports alpha.
This is still building on Experiments with NPR/Toon shading in Eevee
Use guide:
The main shader is plugged in as shown in the annotated area, and can be whatever you like. Change it to another shader as you like.
The amount of coverage is set using the ColorRamp as shown in the image. Move it left and right to get more or less area.
The two (main shader, ink shadows) are mixed with a Mix node (not the mix shader node) so that you can play with the different modes (I have it set to Multiply mode in the screenshot). Change the mode or the factor to affect the mix.
I like when the area is dark enough to see the cartoonish line, but light enough to allow the textures and complexities of the underlying area to shine through.