Agree with Carol regarding Orient Beach, but a few other beaches might work for you. A few that I've been to where you can bring your own umbrella and chairs include Happy Bay Beach (which is just west of Grand Case), and LeGallion Beach which is near Orient Beach to the south. The former beach requires a 15 minute walk to reach the water, but it is a truly beautiful and unspoiled beach. LeGallion faces to the east like Orient Beach, but was always calmer with lighter wave action and more of a family beach. Of course, I'm not sure what it looks like today post Hurricane Irma. Grand Case Beach is very nice, but from my experience was limited regarding places to rent chairs and umbrellas. For Grand Case it is usually best to bring your own stuff.

Just a quick thought if you want to avoid always renting chairs and umbrellas on the beach. My wife and I bought our own large beach umbrella several years ago, and it folds down to just 24 inches so it easily fits into a suitcase. Takes just minutes to set up, and then we throw down two large beach towels to lay on. Many of the beaches on St. Martin have chair and umbrella rentals available, but when you travel to some other islands (i.e., St. Barthelemy) finding such rentals can be difficult if not impossible. Sometimes we do rent chairs with an umbrella, but we always carry our own umbrella and towels just in case. Just an FYI!