Thanks, this was perfectly filmed!
My first thought is that it looks like you are trying to consciously make small movements, like staying close to the string. Would you say that is correct?
Other than that I would say that your motion is a combination of different joints and your body is going back and forth between them. Can you feel that your elbow kicks in sometimes?
The result is that when you’re using your wrist, the motion is mostly USX (it’s actually a bit erratic) but when the elbow kicks in, the motion becomes DSX since that’s the only way the elbow can move. You can see when you play omn the G-string that you start out with wrist but at the end the elbow starts to move back and forth with the wrist. The motion of the pick changes. On the high E-string the elbow is active from the start and you can see that the motion clearly is DSX.
When you play with the left hand, you go back to mostly wrist and get more of a USX motion. It is a bit flimsy like your not holding onto the pick as hard or it swings a little bit. It’s actually hard to see but this is probably not the exact same motion as when you tremolo pick. It’s also of course slower but it’s hard to say if it’s because of the different motion or if it is the problems raised in the pursuit of being in sync with the left hand. What would you say yourself?
My suggestion would be to try tremolo again but this time, don’t try to be “play correct”, just go for speed and see what your body will do. It’s also almost good if the motion gets a little bit bigger because this will actually help with string switching.