How do I set up this automation correctly?

How do I set up this automation correctly?

When contacts enter my Subscribers list, I want it to reference a custom field to see if it is empty. Then I want it to do two things:

If empty:
Assign a tag based on a different custom field. If that custom field is empty, assign a different tag.

If NOT empty:
Wait 21 days, then apply a tag based on a custom field. If that custom field is empty, assign a different tag. 

Notes: I do allow subscribers to re-enroll in this workflow to get assigned new tags if they leave the list and come back. 

This Workflow is the starting point for all my other workflows. See attached picture.

    Nederlandse Hulpbronnen