The only boat I can think of that fits ALL of your criteria is the Hobie Wave.

It LOVES wind, so in a windy area like yours you will be able to sail it a much higher percentage of the time than you would any other beach cat.

It's great and safe for singlehanding because it is super-easy to right -- a 90-pound kid can do it. And it has the bob at the top of the mast to keep it from turtling.

It's boomless and boardless, unsinkable, and easy to tack.

It is 240 pounds, which is heavy for its size but probably lighter than the other boats you are looking at.

It will carry that 500 pounds of people weight very easily.

Its seating area (trampoline and decks) is larger than a Hobie 18. (I've measured both to compare.)

It's a great boat for teaching your kids to sail -- in fact, they will probably take it over and you will have to get another one for yourself.

You can add the optional jib kit and the optional spinnaker and the optional trapeze wires if you want to jazz it up. (But it sounds like in your area you don't exactly need more sail area.)

Are there beach resorts in your area (Indonesia, right?) If so, resorts usually have Hobie Waves as rental boats, and you might be able to get a good deal on a used one. If they don't have Waves, convince them they need a bunch of them, so you can get one for yourself out of their container-load.

There is only one big problem with the Hobie Wave. If you are sailing where there are a bunch of other beach cats, when the wind is 25-35 and nobody else wants to take their boats out, they are all going to want to borrow yours.