Thanks for your help! I did find a way to use the minimax plugin as you suggested to get an acceptable result, all on the one adjustment layer. Part of my problem was that I was isolating all of the coloured area, when I should have been isolating the lines. Below is the result of my isolated lines

And this is the lines on top of the art

Then reducing the adjustment layer with the effects stack by 40%

and the effects stack

The look isn’t quite as nice as if I were to separate the character art in to multiple layers, as the other comment suggested, but this is going to be convenient for most cases I think.
The only thing I couldn’t figure out was how to blur the colour art of the coloured line, without bluring the alpha. Using the Channel Blur plugin didn’t give me the results that I wanted, and instead gave me a black halo around the line art.
Below is the lineart without blur

Lineart with the channel blur

And the composited result
