Delia works fine as a nn for
Cordelia. After all, it's built right into it. I do like
Cordelia, btw. It's lovely, unexpected and slightly mysterious and exotic. I don't care for Cordy or Codi (my new son-in-law is named
Cody so it's strictly masculine to me) although I do think
Corrie is nice.
Dee's cute as well. Other nn options:
Cora,
Lia.