Hey man. In your latest video:
Much better keeping your hips up at the start of the pull. Good job fixing that.
It appears as if your belt is actually causing you not fully flatten your back. Your back extension was much better in the first two videos without the belt. You are either wearing it too tight, or it might be too wide of a belt for you, but it is currently interfering with your ability to get as flat as you would otherwise.
Lastly, you are still bending your knees too much on the way down. It might be helpful to think of the descent as two distinct movements: "Lean and dip." The first part of the movement just barely unlock your knees and lean over until the bar just passes your knees. After it passes your knees, keep your back angle constant, and just bend your knees to finish the rep. If you do this correctly, you will end in the exact place you started with the bar over mid-foot.