Nowadays I don't think most people associate
Sadie with
Sarah, the same way they don't associate
Molly and
Polly with
Mary.
To me,
Sadie kind of sounds like someone who is friendly but not smart.
Sadie Meadow is my favorite of those, it's got a cute happy hippie feel.
Sadie Madeline,
Sadie Juniper, and
Sadie Gabrielle also work well.