We also have some structured projects https://networkx.org/documentation/latest/developer/projects... but they are usually for programs like GSoC/Outreachy.
Feel free to start a discussion https://github.com/networkx/networkx/discussions if you are looking for something specific :)
[I am one of the NetworkX devs]
Imo the best thing is to have a mentor who hopefully can find a reasonable thing to improve/add/fix and can help calm your nerves (submitting a pull request can be kinda scary haha). With NetworkX, in the past there were a lot of minor inefficiencies (things like materializing a list comprehension when you don't need to, etc) that are 1) easy to understand 2) don't require in depth knowledge of graph theory 3) are generally applicable to all python code and 4) easy to measure the results. They aren't the flashiest contributions, but the people I've done it with (6 or 7 at this point) have all seemed to enjoy it.
To be clear, I'm _not_ affiliated with NetworkX, but if you are interested in trying it out, my contact info is in my profile. Feel free to reach out.