I like it. It's much better than
Diana in my opinion because the "th" softens it.
Diana has always sounded a bit too hard to me.
However, I don't think
Diantha is soft enough for middle names like
Lucy or
Florence.
I like
Diantha Cleo/Clio,
Diantha Eleanor/Elinor,
Diantha Harriet,
Diantha Joan,
Diantha Miriam,
Diantha Rosalind,
Diantha Rosemary, and
Diantha Vivien best from this list.