Knee probs from soccer here as well. I'm a fairly active/athletic person and have all but stopped doing heavy squats. Now on leg days I do high rep squats using a bosu ball standing on the platform side. I normally hold a 30-40lb dumbell under my chin for 30 reps. For heavier weight I only do sumo front squats. For some reason front squats feel much better on my knees and the duck/sumo position of my feet when squating feels much more natural to me. I never load up more than 200lbs.