Although they give excellent physical intuition, Feynman's lectures don't exactly cover any "theoretical physics," they're more of an intensive introductory course to physics as a whole.
His QED in some sense helps you understand how QED works, but obviously one wants to learn the mathematics behind QFT, and QED is quite qualitative in nature.
Comments
You can probably do better by going through Feynman's lectures. And maybe also his QED.
Or perhaps by folowing the reading suggested by Gerard 't Hooft: http://www.staff.science.uu.nl/~hooft101/theorist.html
Although they give excellent physical intuition, Feynman's lectures don't exactly cover any "theoretical physics," they're more of an intensive introductory course to physics as a whole. His QED in some sense helps you understand how QED works, but obviously one wants to learn the mathematics behind QFT, and QED is quite qualitative in nature.