[qwrshr]

Another, lower-risk option: at some universities, it's quite possible for undergraduates to take graduate classes, especially if they've had the relevant background. So you could try applying to be an undergrad at such an institution. And you might not need to take that many gen ed. requirements if you find a university with flexible requirements (e.g., google 'open curriculum')

This is likely to be the easiest way to learn more theoretical math-and-CS subjects, though it'll probably be more expensive than just getting into a phd program.