gspurr

The OP is based in Paris (so why limit them to the Caribbean?), mentions considering the Maldives, and states that they are looking for a "secluded, ultra relaxing, beach vacation". Cancun doesn't really fit the bill here does it? I've been to both Ziva and Zilara in Cancun and while I did enjoy my stays I would't go there for a milestone birthday/anniversary if I was looking for somewhere secluded.

I enjoyed my recent stay at the Unbound in Phu Quoc Vietnam (although some others have stated on FT that they don't like the place). It is very isolated and relaxing. You could pair it with a few days at the PH Saigon. There are a number of SLHs in Bali and Thailand that look interesting. With southeast Asia it all depends on the time of year as if you go during monsoon season you get a lot of rain, heat and humidity. And, of course, there is the coronovirus to worry about--but since the OP mentions making a decision by the end of March there is still time to see if southeast Asia is an option.