I realize this is not exactly what the OP asked for, but I'm also trying to make sure the OP doesn't lose sight of the basic physics when he finds himself swamped in difficult mathematics. In this instance, I wouldn't even call the Bohr atom a watered-down approach. It is a very important stepping stone that provides a very important part of the foundation. One thing I think the OP is looking for is some useful mathematics, and the Bohr model of the atom provides it. There is no simple way that I know of to learn the more complex models, including the one using the Dirac equation which I never completely understood. (presented in Bjorken and Drell's textbooks).