First, thank you for all your continued questions and feedback. We really learn a lot by users like you. You might be surprised how often we reference users by name, and their various questions, in our company meetings.
When you disable the email template, that makes the system not create new scheduled emails based on that template in the future. It does not stop the current ones from going out UNLESS you select the "remove from current bookings" option that shows during confirmation. If you don't select that, all the existing scheduled emails based on that template will still go out.
Disabling the template basically means that the system will do what it normally does in that scenario. So if it's a scheduled template, nothing new will be created. If it's a regular template, the system will use the default system message.
There are a lot more options and rules coming to this area of the system. We're planning on refactoring quite a bit of this to be event driven. So at X Event in the system (booking create, booking paid in full, etc), you can define a number of settings: send system default, send x template, send nothing. Likewise with scheduled ones. Also new types of alerts like SMS.
You can certainly create a scheduled email template that sends emails 21 days after departure. However, when you newly create the template, it will only create scheduled emails for FUTURE bookings and skip over any ones that are past. The reason we've restricted this is because you don't want hundreds (or even thousands) of old bookings getting slammed with a newly created template that you didn't intend for everyone in the past to get.
It sounds like you're looking for a marketing tool to send newsletter type content out. I don't think any of our scheduled sending stuff fits that bill at the moment. I suppose if we allowed you to take a template and do a one-time in-place send to all bookings (or a filtered list of them), that would meet your need. Something to think about.
Hope this helps,