I've only done Grand Turk of the places you've been so I don't know how St Vincent/Bequia compares to most of them but they would certainly be different from Grand Turk. Not a whole lot of people have gone there compared to the common destinations but if you search here you will find some reports from myself and a few other people, I don't want to repeat everything. Basically you don't see much big stuff in St Vincent, but lots of cool small stuff including seahorses and frogfish. As far as above water, Bequia might be my favorite island of anyplace so far.
Another combo you could do is 2 different spots in Belize, since there's a number of fairly distinct dive areas - Ambergris, various atolls, southern Belize, and there's several liveaboards. Also jungle lodge type places if you're interested in giving up some diving days for jungle/ruins/rafting/animal type activities.