This is the first time I've responded to any forum post in this site, so I'm hoping someone will correct me if info provided is wrong in any way.
Hiding the assessment in the Gradebook should prevent student from being able to see mark/feedback.
Goto >grades >gradebook setup > setup tab
Locate the assessment where the marks/feedback need to be hidden, (on the corresponding line) click the edit and select hide from the drop down menu.
Click show against the assessment in the Gradebook, when you are ready to release mark/feedback.
Changing assessments marking workflow status to release sends marks/feedback to the gradebook page but the hide/show function in the Gradebook, controls visibility of that information to students.
I just did a test and was able to change the marking workflow status back and forth as needed, currently using Moodle V3.9 but things might behave different depending on your version of Moodle.
I really hopes this helps
LS