This feels like it should be on a phone. I'm not sure how you would dish out the mandatory 3 stars thing that every phone game has to have though.
I don't have much else to add beyond what the other comments said. However, I didn't find it harder at all than it would've been without the theme. I've played enough stealth games to just switch gears. If you want to continue working on this game, that might be something to consider either emphasizing or moving away from. If you want to move in the direction of careful and stealthy, limited ammo would be a good first step.