Artificial languages are where you’d look of course, and there are much simpler languages than Esperanto. Basic English was renowned for having a small vocab. My own favourite, with a comparably small vocab and a much tighter grammar, is Interglossa (as opposed to its revival Glosa).
Natural semantic metalanguage has an extremely small number of concepts, but is parasitic on natural language grammar, and is not meant for communication but for definitions. So it’s not quite the same thing.