Currently, in Review Hub, reviewers can approve, reject, or comment on review items. These events trigger notifications for reviewers and moderators when the last checkbox is selected.
However, there is no separate action for individual reviewer to explicitly mark their review as completed and send a notification such as "Review completed by XYZ" to the moderators/author.
That said, you might want to instead post a comment on this thread, because it sounds similar. I think you may have some differences, but I think you can make the final call on that. :)
