I am a back-end engineer with a pretty solid foundation in computer science, I believe. Topics I'd really love to study are compilers, operating systems, and computer networks. As I understand, these topics are absent in CCs.
Yes, these are all upper-division courses, at least in the context of American higher education. You may be better off looking into inexpensive options for completing a bachelor’s degree in computer science, such as online education, and seeing if you could test out of some of the intro-level CS courses. Unfortunately I don’t know if any specific programs off hand.