Interesting list of desires. Oddly enough, what kept to my mind wouldn't require you to reach the Caribbean; have you been to Key Largo yet?
My trip report from Sept. 2013, about your planned time of year to travel. If you like small boats & don't need no-added charge guides, Quiescence might be your op, instead of Rainbow Reef Dive Center. But consider:
1.) Lots of shallow, fishy reef and easy diving.
2.) Option for morning trips to deep wrecks;
Spiegel Grove, Duane &
Bibb.
3.) Option for road trips to topside diversions; Miami Zoo, Jungle Island, Lion Country Safari, etc...
Not much in the way of sandy beaches, though. Definitely not off the beaten track.
One issue you may face is that when a destination is 'off the beaten track,' there's a reason (maybe more than one). If a place has nice reefs, easy diving and real ship wrecks, a lot of people will want to go there.
Have you been to the British Virgin Islands? I don't know about 'challenging,' but the wreck of the
Rhone is often mentioned. About how many wrecks to you need?
Richard.
P.S.: For my own curiosity, why is Grenada off the beaten path? I'm not arguing that it is; I hear about it very sporadically. So, why isn't it a mainstream dive destination like Cozumel, Bonaire & the Caymans?