A few brainstormed suggestions for both would be nice. They’re both fairly new to my list, and I feel like I can’t think of anything.
My style is all over the place, but to give you an idea a few of my current favourites are: Iris, Ophelia, Greta, Estella, Beatrix, Eilidh, Amelia, Sybilla, Aoife, Rose, Virginia… they tend to be distinctively feminine and old-lady-ish, but not too frilly.
I should mention, Zosia is ZOH-sha, ‘zoh’ rhyming with ‘go’.
As a side note, do you prefer Posey or Posie to Posy? It seems like Posy is correct by usage in names but they’re all used to refer to flowers. It wouldn’t be short for anything.