I understand about the amount of time required for FRC in spring… Good luck with Recycle Rush.
The great thing about math circles is that you will have a chance to meet more kids with similar interests. I know that homeschoolers have plenty of social interaction, but it can still be difficult to find kids who are part of “your tribe” whether you are homeschooling or at a particular high school. Our math circle has been great for my son socially, and he is “only” in Calc BC as a sophomore.
It looks like there are several math circles in the Bay Area. https://www.mathcircles.org/Wiki_ExistingMathCirclePrograms_view
I agree with others that you should check with the Extension programs. Ours was standalone, but has been moved into our UC Extension program, though you wouldn’t know from any website. We don’t have a local CSU, so I don’t know about them. (Hmm, I had to Google why it isn’t Cal State Hayward anymore.) I’d say just send email to people at the Extension department. Someone will point you to the right person.