sorry may be a bit late for this reply but your squat is totally wrong

looking at the video, especially towards the last few reps, you are bending down your upper body first before you bend your knee, as if you are trying to do good morning. thats the problem with using plates under your feet as it pushed your body slight forward, throw that plate away and squat with your feet flat on the floor, also, dont do low bar.

and rule of thumb, your knee shouldnt be passing your toe when you squat

you are skinny so I dont think why you have mobility issue.