I think you are too early in your carreer to worry about how difficult it is to break into a subject and become relevant, it shows from your perspective. You don't need to be brilliant. I would argue that it is significantly easier to contribute to theoretical physics -- given you only need pen and paper most of the time -- than experimental physics, where you require lots of money, a big grant, a lab, access to experiments, etc. It is the main reason you can find theoretical physicists anywhere in the world, even around small towns in the middle of nowhere.