I have the entire drawing flattened on one layer, and on a new layer on top choose Filter>Draw>Perlin Noise (ignore the layer folder on the very top I was too lazy and forgot to move it)

I change noise settings to something like this

change that layer to overlay or soft light

I lower the opacity

then merged the entire thing on a new layer

and edit that with posterization

mess with the numbers

then I lower the opacity of that layer where I see fit

Voilà