Along with the Zercher squats I would say Zercher deads would be an interesting addition. I do those sometimes to simulate a stone pull. Or, go with some good old front squats. There is a good deal of quad strength needed off the floor.

I would say my sticking point is just below the knees. I usually don't get stuck on the ground, and certainly once I'm past my knees everything is cake. So I know for me that a lot of speed is needed to ride through my sticking point, but I'm not sure how you overcome that initial point. Pulling an 18" rack pull is like square in my sticking point.

I like the idea of starting from slightly off the ground (a deep rack pull) and see how that feels. Some deficit work might help, though I often feel like it alters my form too much to be a good tool, but you may experience something different. I pull conventional (not sumo) so because I squat down over my legs there is only so much "down" I can get into before I get in the way of myself.