I think that at an intuitive level maintaining the visual of “downward pickslanting” might be better kept for the actual angle of the pick. Ie–>> the top half of the pick is closer to the floor than the tip. I think the visual is more in line that way, rather than “downward motion that escapes the strings”. Or “downward motion that escapes the strings by moving “up” away from the guitar”. (maybe I’m wrong, but what feels intuitive to me).
I just posted re. the idea that “supinated” and “pronated” could be thought of as guitar “stances” similar to orthodox/southpaw in boxing.
It seems that supinated would almost always being using “downward pickslanting” - meaning the angle of the pick - though as you point out, the degree of slant would vary quite a bit.
Personally, having to choose between “slanted pick” or “slanted motion” I would think “downward pickslanting” would be more easily grasped when applied to the pick itself. The “slanted motion” could be labeled something different (what? Who knows? lol, that’s why there’s this thread).