An Automation needs to be created, or the ability to create custom-Automation, as such:
When all Items (or specified Item) in a Group status changes to Done, then Archive the Group.
Currently, you can Automate the archiving of an Item, but not a Group of Items. I am currently managing phases by Items in a Group. Even if I created a “Completed” Group, then used Automations to move Items to that Group, there is no way to tell which Group the Item originally came from.