Cassandra. It has lots of great nickname potential, great historical and mythological context, and less confusion about the pronunciation. Really you’d just have to deal with “cuh-SAN-druh” vs. “cuh-SAWN-druh”, both of which are very pretty and essentially the same name, just accented.
Cassia would get ”CASS-ee-uh”, CASH-uh”, which I don’t like at all, or “cuh-SEE-uh”. It’s a bit confusing and they sound like completely different names.