Choosing the messages a customer would want
List the messages you would want to receive if you were the customer, and delete the rest. Set the reminder timing from what the customer has to do to be ready rather than from what is convenient to send. Configure your own email address so the message comes from you and a reply reaches you. Rewrite the default text in your own words, including the sentence about what to do if they need to change something, because that sentence prevents the most phone calls. Then send yourself one of each and read them on a phone.
What the appointment reminder app will not send
These are operational messages about a specific booking, and only that.