I tried something last week where I did a 3-second negative, then came up a 1/4 rep and held that for two seconds, then went back down and finished the rep. Awful!
I also tried a “Bulgarian deadlift” if there’s such a thing. I set my legs up like a BSS with one on the bench behind me, but I had the bar in front and deadlifted it. It forced me to extend my knee a little more at the top. I don’t know if I loved it, but it was different.
Agreed. Interestingly, it is the only squat variation I can do that doesn’t annoy my lower back. I have been recently doing them in my half rack and using my hands on the supports to help with a few forced reps at the end. Simple bit of kit but very effective.
5x5 front squats, paused superset with cycling home uphill blows mine up
I had a similar issue, being able to back squat a ton of weight, but my glutes over developed my quads. What made the big difference for me, in addition to structuring some pre-exhaust consideration in my training, were front squats. Once I got comfortable with them I thought my quads engage like never before.
I’m sure this sequence might help.
1- front squats, non lockout, explosive 2/3 ROM with controlled negative
2- single leg extensions
3- back squats, Non lockout for reps
You’re welcome -lol 
S