Sam, the hight of your backswing is not the issue, it is how it gets that high, you are pulling the ball back instead of just letting the ball swing your arm. Opening the shoulders to achieve a higher back swing is not the issue, it is pulling the ball back to get it that high. If it swings that high naturally then that is not a problem.

I think what Desertdog, and demo, were commenting on about the pause in the swing is that you are stopping your arm instead of just letting it fall, as displayed by the numerous times your arm stopped by your ankle at the release, the arm has to start slowing down before this point in order to stop that abruptly.

I think if you try the one step and just let the ball swing your arm and keep things simple, then try a two step delivery then a three step then a four step, each succeeding at least ten good shots before advancing a step, and then start each practice session the same way you will pick up the accuracy needed to bowl with your left hand. As it is now you are spraying the ball from gutter to gutter with no clue as to where the ball is going which is why I suggested to stay bowling right handed. Some people are not able to develop the accuracy needed with the opposite hand, some people are, but trying to jump to the end product before working your way there and building strong fundamentals will cut your left handed bowling success way down, so keep it simple and learn to control the ball before trying to get a high back swing and throw a big hook.

I managed today to get the swing more straighter, and get rid of some of the drifting, atleast the 2nd step goes almost directly infront of the other foot, instead of crossing over a lot to the right.

I think you should try some of the things suggested, they may help you.