I'm not keen on
Joan. I know what you mean about
Jo being a nickname but for a middle name it doesn't bother me too much although maybe a longer name is better.
I like
Cassandra Lydia.
Cassandra Claire is too many Cs for me.
Cassandra Sylvia is too many S's and I don't like
Sylvia anyway although I wish I did. My dad wanted to name me
Sylvia and it would also honour two relatives (not that I'm big on honouring).