OK, one of these days I will make it to Thailand.

With that though, I think the OP was asking about beach locations and none of those 3 Cat 5's you mention are listed as "beach" hotels on Marriott's website. In checking it looks like Marriott lists 5 hotels as "beach" hotels. Unfortunately, 4 of the 5 are all Cat. 7 or above. That means the OP would have to come up with another 80K of points before they could order a Cat. 7 TP with the max miles.

There is one Cat. 4 hotel, Rayong Marriott Resort & Spa, that would only be 270K points. I don't know anything about that resort and specifically why it would be so low a category. Could be due to remote location or could be due to not being as nice a resort as the others. Maybe someone could comment on that resort.

But in general, I would tend to agree that traveling that far for a 7 night vacation can be hard on a person and a longer vacation, 10+ nights, would probably be better. But doing that would require either more points or more out of pocket cost from the OP. Not sure they wanted to spend a lot of money on the vacation (which is why I was focusing on closer beach properties where the OP could focus on a higher Cat/Tier hotel/resort.)

One more beach location, in Florida no less, is the Harbor Beach Marriott in Ft. Lauderdale. It is a very nice property on its own private beach location with a very nice pool to boot. Flying there should be fairly easy and quicker than most other "tropical" locations and you'd get a pretty nice resort/spa to stay in. Plus you would be a quick drive or water taxi ride to a number of very nice restaurants and other activities.