There's way too little information online regarding toon shading techniques in Blender. Why is that? I don't know. But it sucks.
Anyway, this guide will give a few tips and tricks for which render settings to use when rendering these types of illustrations.
Render settings
Shadows
In the Render Properties you'll find a section about shadows. This one is pretty important, since the wrong settings here may create some crazy unwanted results.
You want to change both Cube Size and Cascade Size to the highest possible number, most likely something like 4096px. This increases the "quality" of your toon shaded shadows.
You also want to make sure that Soft Shadows is turned off. Smooth shadows creates a more "3D" looking effect, something that we want to avoid in these types of 2D-looking illustrations.
Color management
The color management setting is something I noticed recently. The View Transform setting is set to Filmic as default. This setting gives traditional 3D renders that "movie-like" look by automatically applying slight color changes in various ways.
However, when it comes to flat solid color renders, we want to set it to Standard. This keeps all colors unchanged.
Samples
I've experimented with different sample amounts to see if it affects toon shaded renders, which has resulted in different results.
A really low sample count (< 25) will cause some artifacts in the render, such as jagged shadows and strange dots and stuff. Why is that? I'm not sure. But I always keep my samples at around 300, which has worked well so far.
Rendering a toon shaded illustration in the Eevee rendering engine takes just a couple of seconds on my ~ 4 year old Asus laptop. A low sample count may decrease the render time to less than a second, but who cares.
Rendering engine
I use Eevee.
Why would you use Cycles? We don't need any photorealism features, we don't need any super high quality shadows and we basically don't need any of the fancy features that Cycles offers.
But I know that Cycles has a toon shader node that maybe could simplify some parts of the material creation process? Maybe. I don't know, I haven't even tried it out. I simply use Eevee.