Nothing replaces barbell squats IMO. I am 57 yrs old and I still perform barbell squats, even with my herniated L-5 disc, and without a lifting belt. But I go much lighter than you do. I perform one set of 20 reps, and I'm toast! But both my knees and my back feel great. I merely do 10 reps of leg extensions for a warm up with moderate weight before I do the barbell squats, and my knees are good.
I see no reason nor advantage in training with weight that has you doing 2 to 3 reps per set, (unless you're prepping for a power lifting meet). You want to get the workout of your life? Find a weight with the barbell that you can only perform one set of 10 reps with. Now do ten more reps using the the rest/pause method between each of reps number 11 to 20. Do NOT rack the bar between those final ten reps. Instead, stand underneath it while breathing. The last five reps are murder, and on some days, you might not be able to grind them out without a training partner barking at you to talk you into each rep, one at a time. It is that intense! Every fiber in your body will be yelling: "Please stop! No more!"
I don't always turn up the intensity like that, but when I do, I have to talk to myself and talk myself into each one of the last 5 or 6 reps if/when I don't have a buddy talking at me. It actually becomes an exercise that's both aerobic as well as anerobic at the same time. You heart will be pounding out of your chest and your lungs will be burning. You can also turn down the intensity and still get a serious workout in by lightening the weight and use a poundage that you can do 15 reps with, and merely use the rest/pause-breathing thing between reps #15 thru 20. Your quads still get a great workout. It's a very old routine/method of performing barbell squats, but if you've never done it, you don't know what you're missing