I was watching a youtube video about a man who owned a backyard nursery in Brisbane, Aus, who stab his shovel into the ground in a circle 2-3 feet off his trees in hopes of keeping the tree smaller. No idea if it works, though.
Perhaps summer pruning after the tree finishes fruiting might be a good idea? I don't have that much experience with mangoes yet, but surely you could prune one the tree finishes fruiting but before it flushes. Summer pruning is a great way to keep trees small because it suppresses growth, unlike winter pruning which invigorates the tree.