After looking through and reading every single skill tree for the third time, I finally found that Illustrious Insight can ONLY be learned if you choose a tree path that specifically states “Learn to CRAFT Illustrious Insight”. There are several trees that I learned that only say “Learn to USE Illustrious Insight.”
This is extremely misleading and has been a huge waste of my time. I searched online and read many Inscription guides and NONE explain where exactly to learn how to create Illustrious Insight. They all just say “various tree paths”.
The best case I can make about my mistake is that this is so complicated that even TWO Blizzard customer service reps thought this was a bug and directed me here.
Sharing this update in hopes of helping other people pursuing Inscription not get led astray as well. This is NOT a bug.