There's adding more panels, and there is wiring for more amps.
Depending on efficiency of your panels, newer models may put out 50% more power in the same space, so replacing them on the same mounts could be an option.
If you have another mounting location with different orientation, two arrays can be put into two separate MPPT of an inverter. Two identical voltage arrays (strings) can be paralleled on a single MPPT, even if different orientation. Multiple orientations gives lower peak current, more hours production.
A 100 A panel may per spec be allowed 20A PV breaker, which should be used at 80% or 16A max.
There should be a few ways to tie in a larger breaker. Maybe just a larger PV breaker adjacent to that 100A breaker in the meter box.
To your 2nd point, exactly. The current 3kw array produces power in the morning until perhaps 1p, but then sharply cuts off bc the sun is shining far more on the other side of the roof.
To your 3rd point, I know that the current 20a PV breaker in the main panel is limiting me, but will PG&E just let me hire an electrician to switch that to a 40a breaker? If not, is it possible to have a sub-panel that any additional PV can feed directly into, thereby bypassing the main panel?
I know that many of my questions are likely not the smartest. I want to use this as a learning experience. Tesla installed the system (7.5kw and Powerwall) at my last house, and it rocked, but since this house has the existing 3kw system, I'd like to learn a lot more about this and even do parts of it myself (though hire electricians to do the high-voltage stuff).