When I started taking lessons, a classical guy taught me there was one true way to hold and use a pick, and for whatever reasons, (he was pretty good, for one) I believed him for a long time, and struggled with it.

I eventually made and patented my own picks, and based on the way I use them, re-approached flatpicks. Different things work for different people. But I have to point a pick a little parallel with my thumb, forward, instead of lateral, to use my wrist for crosspicking. I do tremelo more with my arm, and it matters less how I hold the pick. But for firm loud crosspicking up and down across strings I use my wrist, and have to recommend it from my own experience and observations of great players.

The problem some of us have seems to be when we hold a pick firmly enough, other muscles tense when they need to stay relaxed. That was my issue, and I could play pretty well until the tension made me clutzy. The learned memory doesn't work very well with too much muscle tension, I think. It was frustrating. Maybe something about this helps, I hope.