can anybody tell me what is wrong with my leverages? or any advice.
1) this video is me pulling 160kg/352lbs for reps, but if i add 20kg/44lbs to it, my back will start rounding 2) upper back is rounded on purpose since lumbar is the only thing that sound be straight to avoid any back issues 3) lats are already being activated to pull the bar back prior to the pull
Nothing is wrong, you're just not strong enough to maintain your back position.
I recommend against intentionally doing this. Until you're an advanced lifter this is just a cop out against getting stronger and maintaining better positions.
Good. Activation isn't a yes/no thing. You can always "activate" more to stay tighter. Keep practicing this and you'll get stronger.
Most people have weaknesses from the beginning that make them stronger from non-ideal positions. If you keep practicing with perfect form your weakness tend to correct themselves and you'll get better at pulling with good form. Practice with shit form and you'll keep exacerbating weaknesses and just get better at pulling with shit form (and as a result you'll hit a wall with progress sooner).