Hi NBE_K19,
There are undoubtedly many ways to get a flat look.
Use one light source, set to General> Ambient Illumination only (Diffuse and Specular is off, no shadow). This will take a lot of contrast out of the scene.
Set the main light source to Detail> Contrast:negative value (suggestion: start with -33)
In the example, you will find two project files. Each has a different approach to get a softer light and shadow.
I have set the second example to soft shadows, which is not really my favorite, and needs a lot of attention, as it is just a fake shadow, it might produce problems in animation. Set the Resolutions, Sample, and Bias to the scale an needs of your scene.
The Light dome in file 02, is run by the Linear falloff, adjust this and the Intensity of the light, to get your result.
In the material itself, there is no reflection (given your visual source), but specular. Set it as dull (flat and low) as you need.
To flatten out the aesthetic, the Luminance channel contributes a little bit as well, which will give the overall brightness, but low amounts of contrast.
All suggestions are given with the targets you mentioned in mind.
Project files
https://www.amazon.com/clouddrive/share/vRjjbVl7UP7RQa1xUIaVHDaZMsILL2Gvo2xvcKqZuDw
All the best
