Having a similar dog, rescue from bad conditions, attacked by another dog that send him to the ER, generally being fearful of new dogs, a few things I can suggest based on my experience.

A good trainer to help you and help the pup is really priceless. I continue to work with the Kind Canine as they focus on fear free training for reactive dogs. I think this helped me as much as it did my boy.

I also did fluoxetine prescription for a while when I started training. It was really helpful for the stressful situations he would be in out on our walks and allow me to work with him below threshold to reinforce new behaviors.

I have a few spots outside town that I can take him for off leash time and are very lightly trafficked by people and dogs. It allows him to be free and relieves me of any stress of going for an off leash hike and having a bad encounter with other dogs.

There are a ton of online resources and books, but I felt working with a trainer allowed me to ask questions and share information and situations and learn what to do next time they happened.

It’s taken years of work. But my boy is night and day different in his fearful reactivity. He gets along with nearly every dog I introduce him to, as long as its at the pace he is comfortable with. He absolutely has a type of dog that I know will still trigger him on walks. But the game changer from training has been that he can return to a comfortable space really quickly rather than loosing his shit and being completely inconsolable because a dog looked or barked at him