I feel like I've hit a plateau and have no idea how to proceed. The usual advice is to read more books and write more programs. But ...
Sounds like you feel inured. Perhaps you should try something different---even wildly different, seemingly unrelated to your work. You might learn something that noone else considered, that might apply, somehow, to your work.
I don't even feel like I know what I am good at and what needs to be improved. What are even the ``subskills'' of programming and how could they be measured?
Sounds like you oughtta review the fundamentals. Have you read Donald Knuth's The Art Of Computer Programming? By the way, literate programming <http://literateprogramming.com/> seems to be where the future of software developement is gonna be. Maybe not soon, but the basic idea---keeping all sources, including documentation, together as one file, written for human consumption, and, additionally, computer parsing---is quite compelling, and pramatically sound.