I really like
Cynthia - it feels very graceful and has those delicate fricative 's' and 'th' sounds - and I don't really think anyone these days would naturally use
Cindy as a nickname for it like someone might call any
Alexander "
Alex." Maybe
Cece could work as a nickname, if you really want her to have one? Or
Thea?
I don't mind the sound of
Cindy, but the name itself just feels so dated to me, I'd never use it.
Not a big fan of
Lucinda - reminds me of the witches from Wizard of
Oz.