You asked for a specific suggestion
"anything else you'd like to see specific to the nuts & bolts of spey casting?"
and I have one.
I am a very visual learner and would find it very helpful to have a diagram, basic and not flowery , of an overhead view showing the fisherman, rod tip, grip, current, direction of cast, and landing area in a double spey.
It could be sketched freehand, scanned, and included.
It is too early in my learning cycle to have very good questions but anything you can provide in this regard would help.