Kay used to be my favourite name as a kid, and I still have a soft spot for it now. My mother used to say that
Kay sounded old fashioned and personally I don't think it's trendy despite the trendy names you've listed.
I fear that
Kay is too insubstantial alone and the shortness of Okay to '
Kay makes it difficult to use as a full name. But names grow on other people too and I'm sure a baby
Kay would have no difficulty with her name. At least no one would mispronounce it or mispell it (hopefully) :)