Hey man, 3 things I recommend for hip mobility
1. Hip Flexion - Hold on to the dip bars with one foot on the ground. Take your other leg and drive your kneed up to your chest as fast as possible and extend your leg again (without allowing your foot to hit the ground) and then drive it back up. 2-3 sets of 20-25 per leg
2. Hip Extension - 'Single Leg Reaches' For these begin on all fours and take one knee and bring it to your chest and then drive it up into the air into full extension and continue this constant motion. 2-3 sets X 20-25
3. Body Wt. Squats - These just help you work on your form and I usually begin with my hands in front of me before moving them to my temples on the later reps. Just practice pushing your glutes back.
These are what has always worked for me and I find that you can do the flexion and extension about 4 times a week. Your squat is already looking better, no doubt that you will continue to get stronger and 405 very soon.
Mike Zourdos, Ph.D., CSCS
Florida Atlantic University
USAPL Raw Lifter