I agree it could be easier!
My approach is to duplicate the project as a start, and leave off or remove project members and task assignees and collaborators so as to avoid notifications.
Multi-select up to 50 tasks at a time to mark them incomplete. You can continue other cleanup here then convert the project to a project template, or do that other cleanup after converting to a project template (the interfaces are a little different so you may have a preference).
Hope that helps,
Larry