Ah ok, didn't know you were having some back issues.
Perhaps sumo deficit instead of conventional deficit?
Instead of box squats, try bottoms up rack squats? Set up to the depth of where you would be if you were deadlifting.
For me, those mimic the deadlift since there is no eccentric part of the squat.
I've heard heavy swings are good for the deadlift but have never tried them.
As to the assistance volume, I'm pretty sure I read that Wendler said 5x10 is just a start, and after a few cycles you can do 5 sets of whatever (8s, 5s, 3s, etc)
Without progression, I am nothing.
PAIN IS MY BEST FRIEND.