Several suggestions
1.I love Vieques for a very low key quiet romantic stay. I have not been back since before the Hurricane. We used to fly to San Juan and connect on Cape Air to Vieques. There used to be many small guest houses and rental houses. We would rent a Jeep and find a beach and have no human contact all day. Bring a cooler with you, pick up drinks snacks & lunch before heading to the beach because there are no amenities at any of the beaches. We would rent beach chairs from Blackbeard sports. Do a Bioluminescent Bay tour one night.
2. St. John, again, many quiet and private places to stay.
3.There are lots of very small and quiet places along the Yucatan coast. Tulum has gotten trendy but there are several small romantic places to stay right on the beach. We once rented a two bedroom condo on the beach at Akumal that sounds like your Curacao place.
4. there are tons of small romantic lodges in Belize including some private islands. We’ve done places right on the coast as well as in the rain forest.
I’m sure that there are plenty other options. My wife and I just love a quiet beach setting to relax and recharge. Most of my suggestions came from looking for quiet romantic beach front settings.