I would look at the records retention policy from the business, but it is likely that this approach of putting on a Retention Label on documents that should be retained at the end of the project would be easiest. However, when the file that should be retained is done with edits, and has received signatures/approvals from the leaders seems like a natural point at which to apply the label. The labels themselves may work in the settings to say something like retain for 7 years and then automatically delete based upon the date at which the label is applied.