Sorry I missed these clips earlier! Thanks for posting these. It’s a great selection of shots and makes it obvious what is going on. There is a simple answer to your problem:
You are stringhopping.
Your crosspicking form is not actually the efficient crosspicking movement used by players like Molly Tuttle - it is stringhopping. And it is the same as the motion you use for general purpose wrist-oriented playing like in the guitar clips, in other words, also stringhopping. Your tremolo motion is perhaps something else, maybe we could call it stringhopping, but the point is that it is not efficient either, so it’s sort of a moot point what it is.
Your “tense and locked” clip - that is not stringhopping! You’re doing that correct. Either that movement, or one like it but less tense, is what will break you out of this rut.
Your playing is great, you have plenty of musical and fretboard knowledge, but as a few folks have pointed out, to make things more relaxed, you’re going to need to change your picking motion.
Have you watched the intro broadcast on picking motions yet? Most of the basics are covered here:
https://troygrady.com/channels/talking-the-code/introduction-to-picking-motion/
Do not concern yourself with the difference between pickslanting movements and crosspicking movements yet. These motions we discuss here are the core of all of them. If you can’t do these, you won’t be able to play smoothly at any tempo, whether it’s pickslanting or crosspicking. And great bluegrass mando players like Andy Wood are pickslanting most of the time anyway. There is one type of pickstroke (one!) that Andy does which I would think of as a crosspicking movement more so than a pickslanting one, so you can get quite far with just developing fluid “straight line” picking speed.
General note to everyone:
I feel like we’ve dumped a lot of words on @Mando and possibly confused him! This is a common problem, and we all need to be able to look at excellent and clear clips of the kind that he has posted and know immediately when we’re seeing stringhopping movements. Immediately. No hesitation, no guesswork. And targeted advice for eliminating this without writing volumes.