It looks good. Are you tense when playing? When working on new techniques that are difficult, i often notice my whole body being very tense. Try to be very mindful about that, espsecially all the small muscles in your arms and hands that might be tensing up.
Other than that, i'll share some of my thoughts on alternate picking technique:
What really made me get my technique going was when i realized specifically what muscles to activate to pick efficiently, and i remember that it started making sense to me when i noticed
how Eddie Van Halen does it because it's very apparent how he is rotating his forearm back and forth when tremolo picking. He obviously has very unconventional technique, but the principle worked really well to apply to my own alternate picking technique, and i honestly believe this is how every fast picker does it, fundamentally, even though it can look very different from player to player.
You'll know what i mean if you try to just kind of shake your forearm by rotating back and forth (i think it should make sense what movement i'm talking about by looking at Eddie in the video clip i linked), and you'll notice how fast you can make that movement without much effort at all. Those are the muscles you need to be aware of using to alternate pick and the rest of your arm should be relaxed. The more relaxed, the less discomfort/pain you will experience.
Of course these are just my thoughts and just what works for me and i might be wrong
But i'd really suggest looking into it and see if it works for you!
