I actually think this was a really good thing to have happened. I had been debating about how/whether to answer, because my answer was going to sound so lame. But this helps it.Sydney is a runner.
Today we visited a dog park near my parents that we've never been to. As far as dog parks go, it's glorious. Built within some woods on an embankment so dogs can play but also run through woods and up and down hills.
I decided to actually take off her gentle leader + long lead. The SECOND I took it off, she BOLTED away like a bat out of hell.
She ran around and around and around. She ran near me sometimes, but had NO interest in me talking to her / calling her / coming TO me.
She only ended up back at me after she had tired herself out and the other dog who was playing with her came back when called.
I almost had a heart attack when she was running and I couldn't see her. Even tho everyone assured me there were zero ways to escape (she would have searched it out).
So... we're not ready. I may drop the long leash sometimes, as she doesn't get the same mentality of running when I do that, but ONLY if she's playing with her friends.
What I was going to say is, when she earns your trust, you will know it, and you will know she can be off leash. You were still wondering - and this event proves it wasn't the right time yet (but it's REALLY good it was in an area she could not run from). I know how my dog earned my trust, and I remember the steps and tests I gave her. But what we did is irrelevant to you and Sydney. I do think that knowing her history is really important - she hasn't had you reinforcing a no running away, no bolting mentality since birth. Mine did, which is why I say what we did was irrelevant.
But keep working with her. And take her to safe places where you can test her a little at a time. If she is ball obsessed like mine, always have a ball. I know mine would never have bolted from a ball or someone who would throw it. And I do believe that a dog can be in your control off leash.
And if it makes you feel any better, my dog was not off leash at six months - that trust had not been established. So the fact you have only had her six months DOES figure in. You don't know each other fully to the core yet and she hasn't had enough time to learn. It will come.