Limit Field Entry based on rules

I want to restrict the Managers from assigning any Task without ensuring few fields.
Is there a way to check if Start Date and Due Date for a Task is entered, only then the Manager could enter the Assigned To field.

I tried with event daemon automation, that will clear the Assigned To if the conditions are not met. However, the notification email is triggered to Assigned person. Though the person will also receive mail that Assigned to field is back to ‘empty’, still the damage is done in notifying to the false task assignee.

Kindly, let know if there is a way to address this issue.
Thank you.