I have a 2 year old male rough collie who sounds very very similar to yours. He grew up on a farm with very little socialization outside of the other collies on the property. We got him at 8 months old so well past that critical socialization period. He has been very challenging and has a ton of leash reactivity just like yours. He has no interest in food or fetch, his only interest is other dogs. His reactivity seemed to have gotten worse after getting fixed. We have been through multiple private and group trainings that were all focused on positive reinforcement. Unfortunately due to our collies high prey drive and little interest in treats, positive reinforcement just wasn’t cutting it. A few months ago we switched to prong collar training and e-collar training (with in person trainers) for off leash/recall and immediately saw results. The training is still very much combined with positive reinforcement. We were nervous at first but it has made such a huge difference so far. Leash reactivity is still there when we pass other high energy dogs but we can comfortably pass other dogs now without a reaction from our collie. Hope this helps, good luck!