Manage to shoot a video of my squat


Doing 3x6 65kg at 60kg, I'm suppose to do 3x5 67.5kg but the gym has no 1.23kg plate 

This is my last set so I am pretty exhausted after previous 2 sets. I usually squat barefoot because I feel it is more stable for me, this is also the first time I'm wearing compression pants and found that no improvement on weight lifting.
I notice my hips is hyperextended when going up, it is getting obvious starting from 3rd rep.


Could this be the problem I have lower back soreness? How should I fix this?
Could also be I have weak abs/core? Because I don't train them specifically. I tried Captain's Chair Leg Raise today and only able to do as follows:
set 1: 15
set 2: 10 (i rest a few sec and try to complete 15)
set 3: 4 (i rest a few sec and try to complete 15)

Base on the condition above, how should I fix this?
Should I do more abs exercise to strengthen my core (I can do 2x8 10kg Hyperextensions with no problem, assuming my lower back muscle is stronger than my abs)?
If yes, should I do it daily? I notice I have problem to stabilize myself on a 35kg OHP.

My next squat is my PR weight, on Saturday which is 3x5 70kg. Please help