Caddryn
Pronounced like the popular name Kathryn, it is more sophisticated and unique without suggesting triviality. The double d's in Welsh are pronounced with a 'th' sound. Nicknames could be Cade (pronounced like Kate or Katie), Cadi, Cadie, etc.
I'm going to be nit-picky here-- "dd" in Welsh is pronounced more like the "th" in "then" and "this" than the "th" in "catherine" and "thing."