Like you, I own a 7mm for local diving (SoCal is colder than many people realize!) and wrestled with the question of what to do on a tropical vacation. Mine was a LOB to Turks & Caicos in January. Expected air and water temperatures were high 70s; actual water temperatures ended up being 80-81 but the air was a bit cooler, plus we had some clouds and wind. I took my 7mm and was glad I did; some of the folks diving in 3mm or 5mm suits got cold after 3-4 days of up to 5 dives per day. My cold tolerance is maybe slightly below average; I've dived comfortably in 80 degree water in just a 2.5mm fullsuit, and I can handle 60 degree water in a 7mm as long as the air is warmer. (I did just get a drysuit for local winter diving.) But I get colder on each subsequent dive, even when I bundle up and drink hot coffee during the surface interval. You might find that, by the end of the trip, you with your 7mm are the only one who still wants to do the night dive.
Personally, I think people make a bit too much of the inconveniences of diving an overly-thick suit. Each extra mm of neoprene only works out to an extra pound or two of lead for me; maybe it's different for bigger folks who have more surface area to cover with neoprene. (Though I'm not tiny; I'm a little over 5'9, 165 lbs.) I also have a Henderson (Thermaxx), and it's delightfully stretchy, easy to don and doff and wiggle around even in the 7mm.
As for overheating on the surface, do you know what the whole process of getting into the water is going to be like? On the LOB, we geared up on a shaded deck and entered the water right off the boat we were living on--no RHIB, no arduous hike in full gear. If you're able to keep at least the top half of your wetsuit pulled down until right before it's time to get in the water, you probably won't overheat. But if you have to fully gear up and stay that way for a long ride in the hot sun, maybe you do want something thinner.
If money is no object and you plan on doing lots of tropical diving in the future, a thinner suit might not be a bad investment. But as long as you're not in danger of overheating on the surface, I think the 7mm will serve you just fine.