How wide is your stance? It should be heels at shoulder/hip width.
First rep - don't look back between your feet, gaze a few feet in front of you.
When you take the bar out of the rack, bring your feet together to a regular stance. With a wide stance it's going to be more difficult to keep your balance as you unrack heavier weights. And for heaven's sake, don't re-rack the bar in a lunge position! Walk forward until you hit the uprights then drop down - both feet under you.
You're borderline on depth. Another inch.
Okay, that's the nitpicking over. Onto the overextension....
Slow the video down to 1/4 speed. Watch your hips/lower back at the initiation of the squat. You immediately overextend your lumbar spine - you're almost reaching back and up with your hips. Watch your hips as you initiate the ascent - same thing. As mentioned above, squeeze harder and hold it throughout. This applies to your warmup sets as well as your worksets. Starting with the empty bar, squeeze as hard as you can and hold it, to get that fixed into your head/body. I know this is hard, I've struggled with it as well.
I think your back angle is pretty good.