Hey there @kj93,
For situations like this, I’d just recommend turning this into a campaign. You can segment everyone who browsed the pre-order product and then send them a targeted email once it is live. There also may be an option to make the Back In Stock automation work in your favor for this as well, where people can opt-in to an alert once it is live but that won’t capture the browse group, just those who opt in.
And the option of setting the browse flow to manual and then live won’t quite alert your browse group since they’ll be out of the browse flow by the time to set it live.
Launch campaigns would be the cleanest way to go!