Your lower back isn't rounding. If anything, your ass is too low and your chest is too upright. Because you're trying so hard to get your butt low and arch your back your knees are actually getting in the way. The bar is getting out away from you when you should actually be pulling it towards you.
Use more weight. Stop trying to be so perfect. Grab the bar, rip it off the ground, when the bar passes your knees squeeze the crap out of your glutes, and lean back so hard that the only thing keeping you from falling backwards is the weight in your hands.
Current PRs at 242- 495, 335, 585, 1415