I'm certainly not a coach, but when I compare your video to pros with standard good form, I notice that they reach back much farther than you do, and their X-step is much deeper as well. A bigger reach back gives you more arc through which you can accelerate your arm. I agree with others, and the Beato video confirms it, that your arm stops accelerating, and might even be decelerating, before your release. I like to think of the disc as the lash of a whip. You can't make a whip crack unless the lash is moving its fastest at the moment you snap it. Another analogy might be chopping wood. You want the axe to be moving the fastest as it hits its target, which is actually out in front of your leading foot.
A ball golf swing works the same way. Tons of people let their clubhead ahead of their hands before impact (analogous here to the "hit"), and this causes premature deceleration. You won't hit the ball over 250 yards unless you can keep the clubhead behind your hands until after contact with the ball. I took a few lessons from a golf pro a couple of years ago, and he said the single best thing about my golf swing is how well I keep the clubhead back. My ball golf drives sometimes reach 300 yards because of it. He also teaches his students to hit the ball of their front foot. I could probably bore you with discussions about how baseball hitters hit the ball off their front foot, not between their legs, but I'm starting to ramble.
The long and the short of it is that your hit/snap/release should be out in front of your body, at the moment when your arm is moving its fastest. Accelerate through the release, not just up to it.
Now if I could only incorporate what I've learned from other sports into my own disc golf form.
