I love
Cassandra and it's close enough to
Sandra so I don't see it as a big deal and, if you choose it, it definitely makes sense. And you're right, it works perfectly fine as a nickname. Just a side tidbit - I know a woman named
Sandra whose granddaughter was named
Cassandra, after her. It made me wonder if
Sandra's real name was actually
Cassandra (it's not). Just goes to show how closely linked the names are.
Sandra's perfectly fine just a bit dated in my country. I love the spelling
Sondra.