Technique looks good except that you seem to come out of the hole very slow. Is that just because this is a heavy weight for you or do all reps look like that? You should reverse the weight as soon as you hit depth, that was almost a pause.
A powerlifting belt will probably be better than the one you are wearing. The point of the tapered belts is for weightlifter to avoid hyperextending their backs on overhead lifts, there is a reason that you never see powerlifters wearing them.