Projects would be parent to sub-projects.
Project and Sub-Projects both would be parents to Action Items. It is in the Action Items record where you would select the project, then the dependent list of Sub-Projects.
In the Action Items table, go to the properties for the Related Sub-Items foreign key field. Go to the Reference Field Options section: Conditional Values.
Check the box, "The values in this field depend on selections in another field".
Two selection lists will open.
Match "Action Items: Related Project" with "Sub-Projects: Related Project"
Save the field settings.
In the Action Items form, you will have dependent selection lists between Project and Sub-Project.
In the Sub-Projects table, when you enter Sub-Projects, you will select the Project it goes with. This is what forms the relationships between Project and Sub-Projects.
Let us know how it goes.