Becoming proficient in a language usually involves: - writing code with it long enough, - collaborate in a project based on it, - reading code other people wrote, - keep yourself up to date by following the language evolution, - go more in depth in language internals, compilers, etc...

I wrote C for 10+ years (mostly bare metal FW), yet I am still amazed of how little I know about it. Recently for example I learnt of all the things the dynamic linker does in Linux, how symbols memory addresses are resolved at runtime using PLT, ....

The good point about C is that it can be used for very different kind of projects, so the choices are a lot.