Hey,
That Sounds Great! This article would help you out in deciding the right difficulty path and the Game Flow-Flow Theory in Game Design. However, you can actually have some easy levels first and then slowly ramp up the difficulty by analyzing the situation of the game.
Well, about the UI part, you can slowly try to change it and match the looks with the gameplay as that's what creating an odd impression. Apart from all this, as I said the game is super intuitive to play.