Things to consider when having a single enemy have multiple actions: how much can they do without any possible player input. Can they 100->0 a player with 3 attacks? Can they get into range, attack, move out of range, so on their turns the melee players have to dash and never get to attack? Legendary actions split the bosses turn into multiple initiative steps to allow players to react in between actions.
It could be a cool fight, if the boss gets to do one thing after every player turn (either move, action, or bonus action). It completely changes the fight dynamic and might run into problems for actions that also use movement etc but it feels more interactive than getting hit by an enemy that always runs out of your movent range after every attack.