Yeah, that pretty much describes my dog to a T.
It's really difficult to manage dogs like this in public settings. Bring lots of high value treats out with you and be really aware of your surroundings and your dogs body language. If they start to fixate on another dog (staring), get up and walk in a different direction.
It can be hard to avoid it, but you can at least get them focusing back on you as quickly as possible by walking in a different direction. It might mean you can't be right in the center of the action with your dog for a long time, but be on the outside looking in. It doesn't mean you have to avoid places with other dogs, but you have to find a way to keep distance between you and them.
For example, coming into my training class on the weekend, the two dogs she is familiar with, she was pretty much ignoring while they were sitting waiting for class to start, but going in, there was someone walking out with a Labradoodle she didn't know that was bouncing everywhere and wanting to meet all the dogs. My dog started barking and I immediately gave her a gentle pop on the prong collar with an "AHH!" and immediately walked her quickly to the right and the far side of the room without making a huge deal of it, and she calmed right down. The moment she was heading in a different direction (not toward the other dog) she settled.
I actually had a big proud moment, when I was doing recalls at class on the weekend. She was fairly close to the other dogs when I was doing the recall (when other dogs are doing them, I need to be further away, that is where the low threshold becomes a problem for her), and when she was running to me, she was going past another GSD standing to the side. He jumped up to his feet and she looked at him for a split second, but didn't waver from her straight line back to me and didn't slow down at all until she got to me, so I was really proud of her for not reacting to that.
Little baby steps...