Can't use Charlotte as already one in family. Surname begins with H and is short. I did like May but think too widely used now so would like something else if poss. Also like Rose but doesn't work with our surname, also like Lavender but that doesn't work for same reason... Love Ophelia, but Sophia Ophelia just too much!
As Sophia is quite a popular name, I'd try to pick a more unusual middle name (especially if your surname isn't rare). My cousin is Sophia Elizabeth and so is my colleague's daughter. I've heard lots of Sophia/Sophie Grace's too.
Sophia Emmeline Sophia Madeleine Sophia Julianne Sophia Elowen Sophia Natalie Sophia Margaret Sophia Mary Sophia Marianne Sophia Louise