I agree with Anthony, Stella has doggie friends at daycare thats great, big accomplishment for you and her I think...I'd keep those doggie friends.
She doesn't "need" to be social with dogs outside her 'circle' of friends, just learn to ignore them/deal with them in the area. She doesn't 'need' them in her immediate space
I like your idea of observing the dog park from afar, actually observing ANYTHING from afar, to work on desensitizing her to other dogs out and about.
If there's a reaction, back up to the spot where there is no reaction only observance.
I also teach all my dogs a good "leave it"..
When Masi was very young she was nailed by a few dogs that bullied her,,brought out her defense mode quite fast and I had to retrain / rework that behavior, that EVERY dog she saw was did not mean "I'm gonna get you before you get me"..
Teaching that "leave it", and maturity proved to work. If I'm out walking where I know I'll run into dogs, if I see one approaching (on leash not loose),,even before a reaction I just say "leave it",,she does, we pass, no problem. To her, "leave it" means a few things, leave whatever your doing, or don't bother reacting just leave it alone I've got you covered.
Charging dogs is another thing, you (general you),,can't expect a dog to not react when it's being charged by a dog that they do not know , they are going to either react out of fear or react to defend itself..Masi has high defense drives and tho I've never had a dog actually physically 'attack' her from a charge, I use "leave it" when say, a dog is circling us, and she can shut off that defensive behavior.
All of the gsd's I've had/have, had/have a circle of doggie friends that they interacted with no problems, I never have felt they needed to be "friends" with every dog they meet..
Just rambling here