You can already set a template to be on or off On The Agenda! Simply select which you want in the title of the template (the note part, not the template name)
There is a way to do that, but you will have to create an apple shortcut. Here’s the workflow:
A text input query used as the name of the note, so when you run the shortcut, you type in the name you want
Create a note with the template and the name (the text input query) by using agenda xcallback scheme. You can define its template and decide if it’s on the agenda and give it a relative date.
After you create the note with apple shortcut, go to agenda to assign the date event.
Actually, you can directly assign events to a note with xcallback, but I just can’t be certain where is that event, so I ask you to manually assign it. If you have the name of the event and you are certain that you won’t change it anymore, then it’s possible to assign it during the xcallback phase.