Carys is pr. kah-ris and
Cerys is keh-ris. (I grew up in Wales).
I've never met a
Charis and so would have assumed it was pr. CHA-ris, like
Charity. So, none of them are particularly easy to pronounce for people who haven't seen them before.
Cerys is the closest to the sound you want, so I'd go with that. Or choose a less foreign name with unambiguous pronunciation. :)