If I get the 'butt wink' doing a power squat (low bar, widish stance) by hips and back will be killing me the next day . But doing it high bar narrowish stance a little lower back flexion doesn't seem to hurt me. I still focus on not doing it but if it does on that latter type of squat I don't worry about it. Btw, nice squat, most of the guys at my college rec. center can't even do a parallel or lower squat with that weight. I'm glad I have another gym to go to.
To the OP, here's some exercises for your hams, hips, glutes, lower back in no particular order.
-Good mornings (lots of variety here: stance, bar position, plates under heel or front of foot, chains, bands, squat GM, etc)
-swiss ball leg curls
-stiff legged deadlifts and romanian
-one leg sldl
Of those, box squats and GMs are the lifts that have the most correlation to my squat, but tbh honest I'm still learning what works best for me.