Your deadlifts look better than most. You hold your back together quite well. What you feel as you fatigue is your spinal erectors working hard to hold the back flat. You are not flexing in any noticeable way. The main thing I want to see you clean up is your start. You default to a position that is too far forward, with your hips too high. Before every rep, you have to rock back and drop your butt. I would rather see you stay in the middle of your foot so that your back angle does not change when you squeeze your chest up. Fewer things to move. The hook grip hurts. Sorry. There is no way around it. Both the hook and the mixed grips have their pros and cons. Discomfort is one of the downsides of the hook. Lastly, deadlifts feel awful. The whole time. If they felt good, you would want to do them more. Embrace the discomfort. They only feel worse as they get heavier.