I want to get the moss and soot out of all the nooks and crannies but don't want to chip it. Is this more a function of the pressure, the type of the head or some clever wand-moving technique?
Yes. In fact, it may come down to how good and proper of a concrete job you have. If the cream of the concrete wasn't thick enough, it may spall right off. But, for most situations 3000-psi is the absolute most and I've had no problem with down to 1250-psi. A garden hose is fine if you apply wash or bleach first.
There's no reason to crank the pressure all of the way up and in your quite soggy environment I'd have to suggest you actually wash it and just apply a cheap bleach-based house or concrete cleaner or just household bleach for a few minutes. Then, you can determine the lowest but still most effective pressure and largest tip to get it done more quickly, safely and completely.