I have mixed feeling about Kyan. At first I pronounced it the same as your cousin's name, but when I thought about how I pronounced
Ryan, I realized that it is KYE-in.
Beckett reminds me of the singer of The Academy is... It just seems like a surname to me. It could work as a mn, though, imo.
Thackery just doesn't seem right. I automatically think of
William Makepeace Thackeray. I prefer Thatcher actually.