you're fortunate that you can get away with still putting a load on your knees. i wouldn't sweat it bro. the more variation you can have in the angle of the bench to the sled, and the variation in foot placement, the more you can develope even without squats. personally, i hate squats.. i'm so damn wide in my upper body that it's hard for me to get a good grip without some pain in my delts. lack of flexibility has a lot to do with this also. i barely do squats anymore. we have two different sleds at my gym. one with the sled and bench at about a 110-120 degree angle, with the bench being on the ground. and another with about a 90 degree angle with the bench being in the upright position and the sled being parallel with the floor.

that angle plays a big role in how the load is distributed among the muscle groups in the upper leg. i like to vary my foot placement here as well. wider places more emphasis on the inner leg muscles (sartorius and vastus medialis), closer the front and outside leg muscles (vastus lateralis and rectus femoris). further towards the bottom of the platform and you'll engage the calves a bit more, and further up the platform will engage the glutes a bit more. i like to do 2 sets of each heavy, and switch to the simple movements. i'll sometimes stick with one angle for weeks and sometimes alternate from week to week. it's all about consistency, but variation keeps the muscles growing, and this type of variation provides balance. you can build them quads brother without the squat rack.