Fresh chocolate is like a tender charming cayote.

It's a lot scarier looking than it is... 'sz' and 'cz' are both phonemes, somewhat similar in sound to the English 'sh'. Still not easy to pronounce, sure.

Is it onomatopoetic? The sound of brushing one's teeth?

No, it comes from a common Slavic root of unclear origin chet- or shet- with meanings, in various languages, of "comb", "picket fence", "bristle", "fir needles", etc.