I am trying to understand the complete onboarding process. If I pre-launch a course to start at a certain date, how can I let students know the start date after they purchase during the launch period? Normally this would be included on a thank page.
I'd recommend including this information in the course welcome email. You can also include it in the first lesson of the course (e.g. as a Text lesson), so if they try to access the course that's what they'll see.
Once the course has launched, you'll be able to send a bulk email out to all registered students from your Users page (under Support My Students > Users).
I hope that helps!