I love
Cassiopeia. It's a gorgeous long name that seems unwieldy to me, like
Andromeda, but, I think it can work.
I would pronounce it cass-ee-o-pay-uh.
I don't like the nn Cassy at all.
I don't like any of the mn choices.
mns:
JulietFrancesFernandaFlorenceGuinevereGwendolenPhaedraAliceElodieRubyOpalPearlCeleste