Thanks for actively responding, @J-J-M. The option to set the Preferred Delivery Method to Send Later and Save for all the customers at once is currently available in QuickBooks. 

I know how this type of functionality would greatly help you and your business. I'll share your feedback with our developers for further review in the future updates. 

For future reference, read through this article to help you learn about setting up automatic invoice reminders: Automate invoice reminders in QuickBooks Online. 

Visit again if you have additional concerns. We're always here to help you more.