As you well know, only a few courts are ready made for pickup tennis and only at certain days and times.
You also know most old fart's play mornings, and younger better players play after 4PM, with some exceptions of course.
In the City, Dolores Park, 8 courts, is good for weekend 4-4.5 doubles starting around 9-2, but you better be good at doubles.
Golden Gate Park is first choice, but not great on weekends, because courts tend to get reserved for the working folkes. Weekdays are better, around 11-3, then they get inundated with high school tennis team players.
Marina has two set's of 2 courts. Mostly weekend pickup doubles, starting early around 9-2. On weekdays, they're deserted until 4PM, then it's hit or miss.
Julius Kahn, near Aquello and the Presidio (Pacific) had the highest level of pickup tennis in the city, weekend king of the hill doubles mid day, and weekdays after 4PM.
Too bad City College of San Francisco (8 courts), and SF State (18 courts)are now closed to public play during weekdays. Weekends, you'd have to check out the scene.
Dupont, on 32nd and Clement, have 3 courts, is the practice grounds for Washington High's JV tennis team, but has very high level local players, usually showing up late weekday afternoons.
Best to have wheels, so you can drive to Berkeley's San Pablo Park, Mill Valley's Boyle Park, and San Rafael's Albert's Field for sure thing 4.0+ pickup tennis every afternoon and weekend mornings. About a 14 mile drive at most, from SF.