1. Knee travel: they need to come forward several inches and be just past your toes. Your shins are vertical, causing your back angle to be very closed. I suspect that's why you have a sore back. See figure 2-42 in SSBBT3. The figure is technically showing somebody whose hips shifted back during ascent, but your hips are there to begin with.
2. You're a couple of inches shallow, but fixing knee travel will go a long way toward fixing this. Fix your knees and then check depth.
3. I'd suggest weightlifting shoes now, and a belt soon. The shoes will help support the correct knee travel.
Keep it up.