Another thing that feeds into it is not getting enough exercise. My dog had pano and when he was unable to run a lot and explore, he got more frustrated. I forgot to mention, don’t use a training collar on a young puppy.
I'll second the needing enough exercise comment. Still a puppy so don't go crazy, but it takes the edge off the crazy puppy excited energy and was an absolute MUST before we took our pup into new situations. Or we exercised him just to calm him down enough to work with him.
By 4 months old we DID use a pinch collar and got walking on heel down pretty good before too much socializing with other dogs/people. (Sit and Down are pretty important too)
in puppy class when my boy got to barking I'd put him in sit facing the wall, not allowing him to look at the dog he was barking at. (playfully)
We still put a pinch collar on him for walks, training, and when he will be meeting others. He's happy to put it on because he knows he's going to get to do something FUN!
If actually meeting people is too much right now, walk in less busy areas and change your route to avoid meeting people if possible. I would put my boy in Heel when I saw something that he might react too. if he started LOOKING I'd tell him "LEAVE IT!" and give him a correction. Then sometimes a completely unnecessary turn to my right just to insure he remembers that he is on heel and he needs to be paying attention to what WE are doing. Today we walk past yappy dogs in their yards and he can ignore them.. he gets a TON of praise and treats (if I have them) when he does that.
Don't get frustrated, you have a puppy acting like a puppy. If you're sure your puppy is just excited and playful you can also tell people, "he's just an x month old puppy who wants to play, he's still learning proper manners". Most people see BIG DOG and assume he's an adult.
the Clamer GSD you're looking for is 12-18 months away if you continue working with him properly. My Boy is 13 months and we noticed he is A LOT more clam now. (also gets plenty of exercise daily)