Hi,
Not sure what you mean by a core ConTeXt developer. I think, as with anything, you get better at it by using it extensively. I would focus on a use case of interest and take advantage of ConTeXt’s unmatched extensibility to automate much of it as possible. Then you ask for help when you get stuck.
If you want to contribute to ConTeXt, maybe start by having a look at the source and try to understand what’s going on (should be accessible to you since you are a C developer). If you identify a bug, or want to propose a feature, I am sure the community will support you however it can.
Jethro