I expect the Haredim to fight Moshiach because he may not recognize their way of life. What if he shows up wearing a baseball cap? What the Haredim are waiting for is someone who will essentially stomp out all competing streams of Judiasm.
A republic is a form of democracy but is not a direct democracy where every citizen makes every choice. There has to be a constitution to protect the minority from being marginalized. The king doesn’t have to be a mere figurehead, either. The Torah provides for a bicameral leadership structure where the power is divided between Moshe and Aaron but that doesn’t necessarily mean absolute monarchy. It’s just that what the king says, goes.