You want more leg mass, but are only doing thee sets of squats for your quads? That's what your problem is.
quads grow best with high volume, so try sets of 10 - 15 for your squats. Switching to front squats might also be worth trying.
Use leg curls to get a good pump in your hamstrings too. The GHR negatives are all well and good but leg curls are better for your goal.
Box jumps are a good exercise but not really a mass builder. Adding hip thrusts is a great idea, especially considering you don't have a deadlift variation.
And don't fear the leg press! You can go for stupidly high reps without fear of injury. If you're feeling really masochistic try doing sets of 25. It'll hurt, and you'll hate it, but your legs WILL grow.
So you could try something like this:
-Front Squats 5 x 10-15
-Hip Thrusts 3 x 8-12
-Leg Curls 3 x 8-12
-Leg Press 3 x 25 (ouch!)
I know you said you do a lot of ab work, so you can superset the leg curls and leg press with whatever ab work you want as it won't interfere.
Hope that helps! Remember leg day should SUCK, so don't be afraid to really push yourself.