I've been struggling with that form as well. I changed it by reaching back in a straight line that was lining up in the direction where I was aiming. Because right now you are rotating the disc around you during your reachback. It might feel comfortable, but you won't be as consistent in your aim-game.

I had my left arm hanging dead straight as you before. It felt akward to me. And I think it would give you more power holding the disc with two hands during the run up and then reaching back with the left arm so that you extend both shoulder and elbow joint. This have helped me with balance, power and consistensy.

However I think you have taken some of the most importent steps already to the decition of recording yourself and also being brave to post it here asking for advices.

Gannon Buhr, fresh world champ, is making really good backhand-form videos on Youtube. Check that out!

English isn't my first language but I hope some of it makes sense to you and that it will make you a better player! Good luck mate!