According to the existing file structure, Toon Shading model can actually be implemented as a plug-in. As long as the shader file of the Clearcoat shading model is replaced by the shader file of the Toon Shading model, it can be implemented without changing the cpp file. Of course, you will lose the Clearcoat shading model, but considering that you are already in a project that has a stylized need, that should not be a problem.