To work the way you need, this would need to be added to the external enrollments plugin code so there is a new field for course start date, since a single template won't work in your case.
Have you looked at the code or put in a Tracker request to have this feature added?
A workaround would be to have a SQL script (or PHP) go through after the fact and update the course start dates based on course name, category, or some other information.
I had a site where courses started monthly and were in monthly categories. A one line SQL update set course dates based on category id.
If you don't need or use the course id number field in the external
database you could put the date info there, then have a SQL script update the
course startdate based on that.
I'm sure you have tried to use the course uploads tool too, setting default dates in batches after the fact. It will do it, but it's very manual intensive.