I've seen a few "you build the dungeon before you play" games, but I like the twist of balancing the table. I think currently it is a little too easy, given it isn't difficult to avoid placing monster tiles, but I think iterating on the idea could create something really interesting.
I think it might be helpful to make it so that every tile have a chance of being a monster tile, and the nicer (more paths) a tile has, the higher the chance is. And maybe lower the choices the player has for tile placement. Just spitballing, but I think those could be helpful.