If your back is rounding on your pulls, you need to reduce the weight. If the weight isn't that heavy and your back is still rounding, you need to do some direct back work like good mornings and stiff-legged DLs with an exaggerated arch. Remember that pulls need to be done with the same posture/position as the classic lift - if you load them so heavily that you're rounding and changing the movement, you're not going to benefit nearly as much from them. At this point especially, use the squats to push your general strength and use the pulls to build postural strength/position/timing.
Halting deadlifts would probably be helpful to square away your pull posture and position. Use weights that allow you to keep the back fully arched and remain balanced. Don't worry about doing a high pull, but you can do a pull from the pause position if you want.