Sounds like you could bring a lot of value to a team if you contribute as much as 20 hours per week and considering your experience. I would like to give two points from my own experience so far.

1. Gamedev is quite different from web/embedded development. The question for me has been whether to focus on learning a game engine well or learning fundamentals, like graphics programming, so that I can then learn game engines more easily later on. I've ended up doing a bit of both. I have realized that getting good at gamedev programming takes years even for a programmer experienced outside of gamedev.
2. Most gamedev programmers in teams out there are either like you, good general knowledge of programming but lacking gamedev experience, or they are self-taught on Unity tutorials. For the second group they might not be so familiar with CI, devops, pull requests etc. So you might end up wasting time joining teams where you believe yourself to be the junior but pretty quickly realize you are the senior in many aspects. I think most very experienced gamedev programmers don't participate in teams online, either due to risk of burnout or them having a non-compete clause in their contract. So I would say to be wary in giving up your services so easily in the hopes of learning something when you may end up learning more on your own.

Obviously these are my experiences and they may be completely wrong, but I do come from a similar background as yours (with more focus on systems programming) and this is what I've found so far after 8 months or so of joining different teams and talking with people. Would love it if you proved me wrong though!

Good luck!