Instead of manually sending reminders for stages of the review cycle, create an option to schedule reminder X days before the due date (already setup) to anyone applicable that has not completed their step with optional note. Admins could choose to schedule a reminder to be sent to Reviewees (or Reviewer) 5 days before the due date with one message and another reminder 1 day before with a different message. The reminder would be sent to those in the group that have not yet completed their step.