UC Admission -- Best path?

The other answers had good points but I would add that test-optional doesn't mean test-blind. Just because you don't have to submit a test doesn't mean you shouldn't. If you believe you can improve the score you have, then I recommend you do so. Still, keep in mind what the others said about essays and ECs and don't waste time studying too much if you decide to retake.

You could take online classes at your local community college in your intended major. Many community colleges have agreements with high schools where you pay little to nothing to enroll there.

Catasegura has a point of kinda univest in GPA and SAT but while SAT can always be improved ask yourself are you just applying to test optional schools or are you applying to schools that require the SAT. But I would recommend some at home ECs like a blog that you maintain weekly might be about your field of interest. Or you can do something related to major or just a passion project that you are dedicated too because those are equally impressive.

However the biggest non numerical thing on your admission is essays.

Essays acoording to articles I read can make up 25% of whether or not your admitted so if not practice essay writing at least brainstorm and look at how to write an essay like intro body conclusion etc.

I don’t know about prompts but there usually is iirc a prompt saying make your own prompt and respond.

I would recommend taking advantage of the summer to do volunteering that is related to your future major or adopting a passion project/hobby that will demonstrate an interest in your field. This is because your GPA scores are pretty good and SATs policies are changing for this year.