I want to enable at least some users to modify reviews after they are completed

Would be nice if it allowed the flexibility to support both use cases; one of the strengths of Collaborator is it helps enforce that users follow their organization's review process, and I think that should cover the entire review lifecycle, including post-review work.  I'd prefer the option to configure the extra "post review" state (rather than just include it in "Completed") because often there is pressure to show that the review has actually been completed & findings closed, and you wouldn't want "non-critical" post-review work to slow that down.