flopsy, I think the cashus pronunciation comes from the Roman word cassus (believed to be the origin of the name) and the pronunciation would have been cashus first. But cass-ee-us is fine if you prefer that. People will go for that anyway if unsure. It's a brill name both ways imo. I considered it (pg with a boy) but dh wasn't keen.
Have a girl next and ask on here how to pronounce Esme. It gets ugly.