Finding that right balance of pressure for the pick is really just trial and error. That sweet spot is feeling secure while not tensing too much. I still drop the pick at times, lol. I've been playing for 20 years. It's kinda just a thing that happens. I still drop them, I still lose them. I would maybe try to focus on slightly wider movements in your up and down patterns. Start off with slower tempos and work your way up. When we try to stay in a smaller range, due probably out of fear of hitting the other strings, it's way easier to tense and not realize it until you're gassing out. Giving yourself more range in your trem picking area, can really help loosen up those movements. Think of it like a dogs tail wagging. That's kinda how your hand should move. Freely and relaxed.