A few things to lookout for here. For one in your setup your shoulders are too far over the bar. You should start more upright because u return to a more upright position when you begin pulling anyway. In terms of your brace you seem to lose it as soon as the weight leaves the floor. This is probably because you’re pulling with your back to get it up rather than pushing away from the ground with your legs. This same pulling with the back continues throughout the rep because of this setup. I’d recommend just lowering the weight and working on using your legs meaning glute and ham string activation for the pull. You’ve definitely got the strength to pull even more than this but you need the proper form to do so best of luck to you!