I'm happy to finally wrap up and release this asset pack ^^ especially after a few headaches, like my preferences getting lost for my 3D software so all my scaling and snapping was wrong... But everything works as intended now!

Let me break down the custom URP Shader Graph I used in the screenshots!

The vital thing to get right was the vertex data. 99% of this was fairly simple, but I was scratching my head for a while until that Negate node fixed it...

This setup is responsible for approximating old-school vertex lighting, taking the Main Light's orientation into account, and putting that data somewhere (https://docs.unity3d.com/Packages/com.unity.shadergraph@13.1/manual/Custom-Interpolators.html) we can use:

Here you can see some basic texture sampling going on, and here is where we multiply the diffuse by the vertex data from before - plus I'm using some global variables to tweak all of my materials easily:

And the finishing touch - screen-space dithering, made very easy by the Dither node ^^