I know this is a weird question, but if you took a ruler to that indentation in your bed (while nobody was on it), how deep would you say the sag is?

Looking at the Tempurpedic website, it looks like they warranty it for 20 years, provided the bed rests on their own foundation and frame. Remember that you HAVE to get a new foundation for a Tempurpedic, and can't use your old box springs. But they'll warranty it for anything over .75" of sag, so maybe that would be appealing to you. I have no idea if this warranty would hold up in Belize, but at least they're standing behind their product.

I would guess my own bed has more than that much sag on my side. I have a little less hight and a little more weight than you, so I'm very concerned about mattress sag as well