I also like the medieval sound and vibe! And tennis-playing suburbanites from the Betjeman era.
If I had to rank names, it'd be first
Joanna, then
Joan and
Joanne nowhere really. Which is odd, come to think of it, since I far prefer
Anne to
Anna.
Joan Louise is a good one.
Joan Patricia too - very tennis-party. And I know Joan-Ida, who just goes by
Joan. Her
Ida is pronounced with an EE, not an IE. Maybe Joan+vowel might be worth examining: Joan-Alice or Joan-Amy, Joan-Ellen or Joan-Elize, Joan-Olga? If you're heading for ln hyphenation, perhaps not, but otherwise it could work.