My path from intermediate to advanced (and I think there are big differences in vim between these two) was by watching Derek Wyatt (https://vimeo.com/user1690209/videos) do his magic, and reading articles here as well as on reddit. From time to time there are cool blog posts with new things to try. It's also worth to read the source of some of the more famous plugins together with looking up stuff on :help.

I definitely would not suggest to "just keep going". Improvement is slow by yourself and the things that make a major impact are not in your repertoire because you didn't even think they exist or that you could use them that way. Find people like Derek, who are admired by the vim community, then study their works. It's not about mimicing them but about understanding how they solve problems and thereby increasing your thought limits. Then you go and build your own skills with your new horizons as new limits (which is the lonely and hard part, but necessary).

Once you are certainly above the threshold of intermediacy this method becomes effective. Beginners should follow tutorials. Experts have no other choice but study other experts. In intermediate level you have the luxury to get value from nearly all methods (basic courses, tutorials, videos, discussions, studying masters are the ones I know about).