I've finally completed the rigging on the three characters and I'm pretty happy with how they've turned out.
mia_material_x Shaders with Outline
As I was posing them, I had a quick look at how Surface Shaders would look in place of the mia_material_x Shaders that I've primarily been working with, and I'm pleasantly surprised by the outcome.
Surface Shaders with Outline
The flatter Surface Shader just seems to work way better when it comes to representing the painting (something I probably should have realised earlier, but I'm happy to discover either way). Whilst modelling the remaining environments, I'll experiment with using them for those too and see how it all comes together in the end.