Personally I thing the way forward with this is to make it look like a cartoon in post.
I have a little accidental experience with this topic. What (I think) you want to do is like Kadri demonstrated. That is, make things with flat single colors. Use objects that were built to have a cartoon feel/design. Have no volume in the Atmo.. No volumetric lighting.
No shadows! Use hand drawn billboards for the background.
And then after you render out the scene/frames. Use Photoshop plugins on each frame to add the particular cartoon style you want (batch process the frames several times to build like you do with layers)
But really. Terragen really does not do "old style" cartoons well at all (I would say). Modern extremely high production value 3D cartoons, YES!
But old school 2d look, no. It would just be so much work, working against the thinking that TG was built on...IMO.
What you want is something like
https://www.toonboom.com/products/harmony-stand-alone*I guess*
Hope that helps a little. Or at least gives you some more ideas.
