My suggestion was going to be to not use Toon Shading at all, but to use the Diffuse Shader "Gradient" plugin setting, which does toon shading but without the lines.
Note that in your render settings you have to have "Plugin Shaders" ON for a Material with that to take effect.
However in testing that, it worked but I also found the "Gradient" choice would not survive a save and reload, so I've made an AMReport on that:
GradientSettinsLostH.mov