Nice PR Spart. BTB is on point. Lead with the head and keep your chest up (broad-flexed chest)!!! This will help correct some of the rounding lower back.
As for the hips moving before the bar, two suggestions:
1 - Start off at a higher position.
2 - On the start of the lift, think about pushing your feet through the floor.
The higher hip position from the start is a stronger position to get your legs involved with the lift. Much like a parallel squat vs. a quarter squat. Quarter squat is stronger because the leverages are better (short ROM too). Start to low with a heavy DL and your end up having the hips fly up.
Now with that stronger start position, thinking about pushing your feet through the floor will get you legs involved and insure that hips don't rise to fast. These two suggestions along with the lead with head, keep chest up, will get rid of the rounding lower back.