The height of your backswing isn't the problem. Look at your slide and release. You're done sliding before your arm comes down from your backswing causing your finish timing to be late. You actually pause at the top of your backswing. I use to do the same thing and it was hard to break that habit. The pause also causes you to muscle the swing forward.
I couldn't really tell how far you were drifting. 3-5 boards is normal. Instead of fixing it you could just adjust accordingly if the drift is consistent.
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