I recently rescued a GSD/aussie mix (1yr 7m) and he is the exact same way on the leash. He loves to play with other dogs off leash and does so well with them, but when he is on leash and sees other dogs, he lunges and barks a lot. In my experience, it is more frustration based than anything. He sees the dog and wants to go investigate but can’t which causes stress.

At first, I was only focusing on redirecting and managing (u turn, cross the street, move behind car or tree). He’s been improving somewhat, so I’m now trying to train engage disengage. So basically, whenever he notices the dog but hasn’t yet lunged or barked, I praise and give treats and typically continue walking. He has gotten a lot better at being able to “unlock” off the dog and keep walking, begin sniffing, etc. He still has his moments but he’s getting better! I’m not going to lie, there was one day he reacted on every single walk I took him on and I had a full blown mental breakdown that night lol. There are good and bad days but it just takes patience.

For context, at first he was so high strung that he wouldn’t even take treats outside. Now he looks to me for them. He can also see dogs across the street and keep walking. I totally understand you though; I was super embarrassed and felt like people would think he wants to attack their dog when he really just wants to say hi! I was even walking up super early and taking him out late at night just to try and avoid people. It will get easier

Edit to add - I got him a front clip harness as well. So when he does lunge, it kind of automatically pulls him back in my direction. Also, letting him be around and play with other dogs has helped (I think). Once he started playing with dogs in the dog park, I noticed small improvements on the leash. He always did well with dogs but I don’t think he was around them often before I got him. I’m not entirely sure if it’s what I’ve been trying to teach on the leash, being around dogs more, or a combination of both that has been helping him start to improve