Excellent article and I feel you covered the points beautifully. Completely agree with you, which was perfectly summed up in:

“The important point here is that the supplemental not be essential to the understanding of the story. It expands the reader’s experience but is not a necessary crutch for the journey to continue.”

Everything I write and craft is linked together, very similar to the Discworld series. The stories stand on their own, but as a cartoonist and an avid reader of fantasy myself, I love having the option of more material. That’s the key, isn’t it? Providing the further magic for your readers, to the adventure never ends if that is what they choose…

“…my experience also makes me think that to some extent this depends on your audience and their reading habits and what they actually like to engage with.”

Spot on. Especially when it comes to age. Younger middle grade tend to look for visuals and you also have choose-your-own-ending as another example.

Just my meager 2 cents.
Thank you for sharing your considerable insight!