How to relate a child record from one table to two parent records in different tables?

I have three tables: Projects, Tasks and Documents. Projects is the parent table for both Tasks and Documents, and Tasks is a parent table to Documents. In practice users may add documents to the Project record or they may be completing a task for that project and so be in a Task record and want to add a document associated with that particular task. 

I would like documents that are added to a Task record to also appear in the documents list for the Project record so that users don't have to drill down into the Task record to get/search for to documents added that way.

I created an automation that copies the record ID in the Related Project field from the Task record to the Related Project field in the Document record, and that works - but it will only fire if some data changes in the Task record - not if the user simply hits save after adding a document to the Task record.

So I then tried to find ways to force some data change in the Task record to force the Automation to trigger but so far nothing has worked.

This seems like useful functionality so I thought for certain there would be instructions online already but I've been looking for quite awhile and haven't gotten anywhere. Any help would be greatly appreciated!

AJ