Thank you both for your input about dog parks. I'm well aware of the downside of dog parks. I've learned that I have to be cautious of both problem dogs & problem owners. Every visit can be a different experience & I'm prepared to leave at the first sign of trouble or a bad mix of dogs.
Cassie meets other dogs during her obedience class, but it's on leash. At the dog park, we actually met her sister, from her same litter. Play at the dog park, with her sister & a couple of other GSD dogs we met, has gone well. Cassie has gone from being afraid to play, to cautious playing with some other compatible dogs. In my particular recent experience, Cassie has gotten along well with some GSD's at the park.
I am aware that there is a potential for a problem, with every dog we meet at the dog park. Unlike many other owners, I don't glue myself to a cell phone, sit my rear down on a bench & ignore my dog. I'm viewing her at all times & along side of her as possible (except during running).
I might check out some pay-for dog play facilities at some point in the future, but it doesn't fit into the budget right now.