Sore back aside.
When setting up for the lift, push hips back first, then bend at knees.
By doing this you load the stronger muscles, posterior chain, creating tension which will make the pull smoother than trying to lift with your legs.
Imagine pushing your butt back until your torso is almost horizontal with floor. Then bend knees, attempting to have vertical shins, shoulders in line or behind bar.
Initial breaking off the floor is quadriceps, pull bar into you and back, using your back muscles, lats.
After breaking bar from floor: Below knee, strong hamstring contribution, at knee and above knee, glutes will finish the terminal hip extension.
Try looking down a little bit more instead of looking straight ahead. Going into cervical(neck) extension can happen on maximal attempts but shouldn't be regular technique.
You have happy feet. Which means they move between reps. Don't do that.
If your not doing a maximal attempt ditch the mixed grip and save your biceps. Depending on goals straps/hook grip could be better options.
And if, IF, you have any control over it, don't know your gym, ditch hex plates, move to round plates.