Would this work?

  1. Set up a daily recursive workflow that searches for Approval Tracking:sorted by created date:last item
  2. Check whether it’s approved or not
  3. If yes, stop running, if no, schedule API workflow to send reminder email.

If there could potentially be multiple Approval Trackings awaiting approval, you could remove the :sorted by date, and simply return a list of all unapproved Approval Trackings.