For the best-known preschools, e.g. Pacific Primary, Holy Family Day Home - yes, it is too late to apply. In general, most openings at preschools (both the well known and not so well known) are in August/September because that is when the oldest kids leave for kindergarten, making space for younger kids. I agree that is is best to pick a preschool near where you live - driving is a total pain here and walking or cycling are often better options. Without knowing where you plan to live or what you consider a safe area, I can't suggest specific preschools. But I would encourage you to google and start the online application process for as many schools as possible right now. Bright Horizons/Marin Day School has many centers and you might look at them. Also, you might want to join the Golden Gate Mothers Group ($75) for more detailed information.
My son is only 2.5 so I can't really answer your question about the age at which kids learn to read and write here. Regarding curriculum, if you have a preference for the type of philosophy/curriculum you prefer -e.g. Montessori vs. play-based/Reggio Emilia vs. Waldorf - then that will help you narrow down your search here.
If you can't find a preschool you might choose to hire a nanny if you can afford it. Rates are about $16/hr for one child and $22 for two children. Many families share a nanny with another family.