Another reactive rescue

My gf and I rescued a 3yo male Australian Shepherd from the humane society about a month or two ago who had been surrendered from his previous home due to “not enough time” and now I understand why. I’ve grown up with this breed, and have trained them before so the hyper energy and need to work isn’t new to me. Coming home from the shelter we brought him into a pet store with other dogs and people present, 0 problems, as well as when he met our families and friends for the first week or two, great! But after that period I started noticing when he would see anyone in our apartment building, inside or outside, he would lunge with a very aggressive bark at both people with and without dogs. I have been working at positive reinforcing for not reacting and distracting, and have had multiple times where complete strangers have walked within feet of him without a single sound. However, it seems to be now reversing and getting worse again, and he hardly even checks in with me anymore. And when a reaction starts, it doesn’t end until I’ve dragged him away still barking and lunging. I’m a firm believer in positive reinforcement training, just unsure if I’m missing something here and any possible tips to add-on to my training.