First pic notice the lean across the yellow line which is your center and you have already reached your max backswing before the pros and your posture is already compromised. You want to turn, not lean back, it will feel much more upright. So you are beginning to move the disc forward while the pros keep the disc in the same position until the next pic and continue to turn hips/shoulders further back. That robs a lot of acceleration during the throw. Your rear heel is on the ground putting your weight behind your heels and into a non-athletic position that is really hard to recover from.
Second pic is the beginning of weight transfer and you have already turned your hips/shoulders while the pros stay back. This is a major power leak and sequencing error. Notice your shoulder ahead of the yellow line and the pros shoulder is inline with the yellow. You want to stay turned back until you feel more weight on the front leg, then throw. Your weight is also too far behind your heels here, it wouldn't take much to push you over on to your butt.
Third pic is the hit, your front knee is collapsing and your weight gets to the outside of the front foot. Keep your weight braced against the inside of the foot/leg. You are correct about not getting the elbow forward here, or getting extension. Your upper arm gets pinned against your chest or hugging yourself. You need to keep the elbow more forward throughout the throw and turn the hips/shoulders more back and forth. That will feel like your reachaback is very wide or not reaching back at all, but you need to trust it. Once you keep your weight braced on the inside your can release/whip the whole arm more forward instead of just your wrist.
Last pic in the followthrough basically shows your over rotated and weight too far forward. Because you aren't properly bracing your weight to the inside of your front foot/leg, you aren't getting much of your weight into the throw, your weight keeps moving forward instead of being behind the disc countering the throwing action to propel it forward.
I'd recommend you watch/read these, pretty much anything will help at this point:
https://www.dgcoursereview.com/dgr/forums...php?f=2&t=9959
https://www.dgcoursereview.com/dgr/forums...hp?f=2&t=13291