Hey Jack
Let's start keeping follow up form checks for the same lift in a single thread so it's easier to refer back to what the previous feedback was. Or at least link the older thread here.
You are correct about getting more slack out of the bar. When warming up, you should be setting your back so hard that the bar floats off the ground just from the back going into extension.
As far as not pulling around the knees, getting the eyes locked out would help. Then making sure the bar is over midfoot and you're balanced on midfoot. If you rock back to your heels, you're more likely to pull around the knees. Then some cues to think about are...
Drive your midfoot into the ground.
Push the ground away as you hold the bar.
Leg press the ground away.
Stay out over the bar for the first couple inches off the ground.
Peel the bar off the floor.
Squeeze the bar off the floor.
Your back has a little bit of flexion, but I wouldn't stop you from pulling. It needs to be addressed and it can be by attacking the two points of feedback - get the slack out of the bar and stop pulling around your knees.