Wanphattkat - I have dove in several of the same places as you, most recently was PNG, and oh boy do I want to go back - loved it! Living in Colorado, it is sometimes easier to get to the South Pacific than the Caribbean; However we have really tried over the last few years to discover the Caribbean and Latin American areas. I have a similar want list to you, no cruise ships, not a backpacker joint mid to high end accommodations, unspoiled, rich in culture, great diving, and inexpensive sure is nice lol - I am longing to locate that little undiscovered gem. And maybe the search is what is the most fun!
I did discover an amazing place last year, it was not exactly the easiest to get to, but not bad either. It is Bequia - located in the Grenadines. Wow, the four of us came up from our first dive and all agreed the reefs looked healthy and we had made the correct travel decision. I have dove in CR, Roaton, Belize, Mexico, and if I was to do it again would choose a live aboard for those areas. Those places are just too easy to get to and over-dove IMO. Although the Turneffe Islands in Belize were nice. The reason I stumbled upon your post is I was searching for info on Providencia - a Colombian Island. American Airlines and United both have reasonable airfares to San Andreas - it seems totally doable and completely unspoiled. This may be our next dive trip location. Good luck on your search, and let us know what you ultimately decide and how it went!