with the grade for submission and grade for assessment set to 0
Do I get it right that you have the grade for submission set to something and only the latter one is set to 0?
Once the assessment phase is over the students are not able to see any assessments
Correct. You as the teacher are supposed to switch the workshop to the Grading evaluation phase and let it calculate (and aggregate) the grades. If you are happy with them, close the workshop (by switching it to the last stage). The grades as well as received assessments are displayed to your students in the Closed phase only.
Should I have used that strategy instead?
It really depends on what you want. As you are using rubric, you probably want some grade for submission to be received. You can't achieve that with the comments strategy, it purely for getting feedback with no grade.