You are setting up with the bar slightly behind mid-foot and thus your balance is back in your heels during the reps. I would take a slightly wider grip so you can get your knees and the bar forward a bit more (think knees flush with the front of your arms). In terms of the belt you can play around a bit with how high/low it is positioned on your torso, trying to find a more comfortable position. Test these in the setup before doing actual pulls. If it's still a problem you may need to switch to a slightly wider stance to make more room for your torso+belt.