thanks for giving me an excuse to reminisce.
I haven't had that many GSD, but now having a non-gsd (bc mix) I can see they are VERY different. GSD are like living with a mature adult human being, or young adult, 18-24. I know that's a fuzzy statement, but that's how they feel to me--they just seem mature.
In science terms, they display far less neotony in both looks and behaviour than your average domestic dog. Neotony--domestic dogs look and act like immature wolves/puppies, short faces, biddable, etc...
Working lines are a whole lotta dog. They are bred to be high in Fight vs Flight. That doesn't mean they want to eat your neighbour, it means that when pressed or threatened, a good dog will stand it's ground rather than flee. That means that when things go wrong in terms of socialization, you may end up with an aggressive dog.
A good wl dog should have solid nerves, which means nothing much scares them, so if well socialized, that 'fight' drive shouldn't be a problem, but if someone makes them nervous, you'll need to have solid control.
A good gsd is a dream to train, fun, fun, fun, they want their toy, they want you, they want ACTION. But they make good couch potatoes too at the end of the day. They want to be with their people, and they want all their people in one place (so separating off the group will BUG them).
My wl gsd (Dynamo) LOVED agility. She wanted to get things right, just because. She never 'cheated' jumps, etc. working with her was like having an equal partner...but, if not otherwise directed, she would make a game of intimidating other dogs, she could be bossy (I did a lot of obedience and tug with her while waiting during class so she wouldn't have a chance to amuse herself).
GSD's can also be deliberately wilfull. That means, disobey just to see what happens, like running experiments on you. A trait I loved, fun. But now that I have a non-gsd I see why some people don't believe it's possible. My current dog would not dream of deliberately disobeying, he's just not going there. If he disobeys, it's due to something else being more compelling.
I can see how some folks would find a gsd like that challenging, but it was like having an adult partner with a keen mind. Not an easy dog to own and train, and at that size and capability, a huge responsibility.
I don't think the extreme show-lines would be built for agility. Working lines are built for agility, will have a lot of drive, will be very willing to work for you, but keep you on your toes.
A responsible breeder should be able to place you with the right dog. If you do go for a gsd, avoid any that show fear, or nervousness, and be sure to socialize, a lot. They don't need to be friendly, but you really want them to be neutral to strangers. If they treat strangers like rocks and trees, all good.
Given your dog owning history, I think you'd be fine, so it's a matter of what you want.