A bit of filming advice for next time, have the bright light from the windows at your back if you can. It would be easier to see smaller details if shot from the other side. Next time.
You're keeping the knees back when you start and squatting on your heels. Let the knees go forward and out as soon as you start. Lean over, shove the knees out, and squat on midfoot.
Try taking a bigger breath into your belly when you brace for the valsalva. And be deliberate about getting the trunk tight before every rep. Lifting the chest before each rep to get the upper back tight will also help with making sure the lower back is tight.
Could be just the lighting or video quality, but it doesn't look egregious. It may actually clean itself up with heavier weight, as this particular problem tends to do.
Get some lifting shoes as soon as you can.