Seraphina is gorgeous (I prefer the look of
Serafina) and I don't think it's any more pretentious than other long and flowing names such as
Arabella or
Anastasia. It's too pretty to shorten to
Sera (because it sounds like
Sara), but if I had to go the nn route, I'd use
Fina or perhaps even Seri.
Sadie could work, but it would be a stretch, imo.
"Nobody cares how much you know until they know how much you care."
Theodore Roosevelt