If using email with start-due day it will repeatly send the email many times on that day if set the email trigger when day arrives (Many case I have to do that). So I think you should made time for setting the start-due day in the automation. This will help the email send one time only.
Moreover, for example: your first automation in a list has set the start day for the task.
Then the next automation want to clear that start day, and set the due day.
Now, there is no feature to help to clear the old start day, that make the calendar full of that task from OLD start day to new due day. So, please make that feature.