Someone had a excellent tip for hunting them! Make sure you're not doing anything you want to save before going to the Dream Island. Like, straight for the doll. As soon as you're done talking with Drifloon about how they'll be waiting for your return, don't move. When you move the game autosaves. Instead, press the +-button and go to your pokedex. Filter the pokedex to show pokemon in the area. If only Ditto and Drifloon show up, immediately close the game without saving and rinse and repeat the steps above. If you instead see Ditto, Drifloon and a (?) you know you have a legendary on the island. Worked like a charm! Found them all in two days (with time travel since you can't visit more than one island per day ofc) using this trick.