Looks like you've stumbled into color scripts.
https://www.amazon.com/Art-Pixar-Complete-Colorscripts-Expanded/dp/1452182787
Might be in this book, but I haven't heard/seen anything about his actual, sit down process to do storyboard.
He does release the storyboards after the release of each film in japan. I have two of his books. The details are typically character with a little background. Pencil, pen, and watercolor to capture the general stuff. When the actual scenes are made for animation, they add in the detail and make everything much more complicated.
You could try importing the script into Storyboarder and make notes in the caption fields and/or edit the .fountain file directly as needed. Might be a bit turned around from the current approach, but more direct in the end, as you can go strait to animatic. That said, your approach is how I’ve stated boarding, and story Storyboarder seemed like the next logical step to avoid redrawing my thumbnails sketches.
I use a huion tablet with storyboarder (free) Here is a link: https://wonderunit.com/storyboarder/
I'd just like to say that I am an animation hopeful and a total beginner, the industry standard is a Wacom Cintiq and Storyboard Pro by ToonBoom. Cintiqs are SUPER expensive though