As I am using Houdini for flat-look motion graphics instead of Cinema4D, I wonder if there are smarter settings on Mantra (perhaps 3rd party renderers) to render the scene without any shading quickly.

Would someone tell me the idea for that?

Houdini ships with an OpenGL render node, and Wren (a Freestyle type edge render system). Try dropping those nodes down, in the /out context and read their help cards.

If you have access to Redshift, you can plug textures directly into material outputs, bypassing the shaders and lighting. This technique is the fastest way to render any textures, that no longer need to be modified by lighting.

faster way than what? you didn't mention what you are using currently
- one super fast way is to use Constant materials
- another is for example if your scene is already set up with shading and Principled Shaders, to just remove all lights from Mantra and export ‘basecolor’ or ‘diffcolor’ AOVs

I tried all of the methods and realized Redshift was the fastest in my environment.

Thank you so much for your advice!