I like Cassie. I prefer the C spelling… just for how it looks I guess, I almost always prefer C spellings.
I’ve always thought Catherine would work. I also like Cassie short for:
Cassia
Cassiopeia
Callista
Cassiana
Catherina
Carissa
Just Cassie feels more classic to me (where I’m from, Cassidy was a common/trendy girl name about 20 years ago, so I know of multiple in my age group and none any older or younger)