Note that decoherence is not an absolute. It's the degree of interaction/entanglement of one system with another, usually much larger system.

When, to the outside observer, does that conscious experience happen?

Until you interact with a system, you don't know anything about it.

Which means we're all p-zombies until a specific point in the future when we decohere sufficiently to have consciousness up to that point.

Not necessarily, "simply" emitting photons in an irrecoverable way would be sufficient for internal conscious experience. Of course the term "emitting photons" is only defined with respect to a system that can interact with these photons.

Maybe it's a gradual process where the degree of consciousness rises with the odds of emission, even if there is no one to measure it. Or something.