You already mentioned Key Largo, which offers shallow reef diving over a flat bottom (but also deep wreck options for those who want them). Bonaire shore diving would let you cruise the upper strip of a sloping wall, and there's a site called The Cliff that'd be really nice for this type of dive.
One I haven't seen mentioned is St. Thomas in the U.S. Virgin Islands. I've only got 4 dives there. 2 Were referral OW training dives (max. depths 25 and 35 feet deep; no idea whether they were named sites or not); 2 were recreational dives on a cruise stop, at Armando's Paradise (max. depth 43 feet) and Navy Barges (max. depth 39 feet).
I think roughly 40 feet max. is more practical than 30 if not shore diving, but diving over a flat 'hard bottom' may be more mentally reassuring than over a sloping or vertical wall.