Will be hammering abs/low back more often from now on.
Been doing these with a weighted barbell, and loving it.
Although KlokovD's variant seems cooler (I tried it, mother of god difficult to get the weight onto your traps!)
The limiting factor in your squats appears to be glute/hammy weakness. At that midpoint in your squat where bar speed slows, you should fire your hips and glutes to hit the lockout. You are on the right path picking good mornings and/or GHRs as great accessories to focus on strengthening these weaknesses.