How to bridge into programming networking operating systems

I have a decent background in Unix, network and programming and a bachelors in CS. I currently work as the lead network and systems admin for a medium sized ISP. I have about 7 years of experience in networking and a bit more in programming, but mostly higher level languages. Ruby, Python, PHP, bash, c#, etc. I’ve always wanted to dive in and learn to work at a low level in a way that could be complemented with my current knowledge. By that I mean I’d like to work on a network operating system. Cumulus, IPI, DANOS, FRR, SONIC, etc all seem like good candidates, but I can’t find a good starting point. I’ve looked over some code from FRR and it’s just difficult to dive in on such a complex project. I’m highly driven and a very fast learner, I just can’t seem to find much information on where to start.

For those of you in this environment, how and where should someone with advanced networking and decent programming start on the path to writing code for NOSs?