Patio11 started working on Appointment Reminder. Obviously I don't have any insight into that phase - as I was not in the picture then - but patio11 covers it in his IH article here : https://www.indiehackers.com/interview/appointment-reminder-2b8720ca25.
My intention with posting here is to help people who are interested learn "What happens next after I sell my IH business to a new owner...? Does it fall apart? Do they care about my customers like I did? Will I ever hear what happens to my baby?"