Hello Dan, the restriction is applied by the current version of the assignment, and is not fixed. So you can control the release in the way you describe.
As long as your students do not have "In Progress" attempts at the assignment, then you can edit it. When editing the assignment, untick the "Restrict Feedback Until" option, which you had previously set. Then all students who have previously attempted the test, will now be able to see their Feedback via the Gradebook. You can toggle the setting on and off as required, as long as you have no In Progress attempts and the date/time of release is set correctly.