at the moment "allow multiple attempts" doesn't mean what I think you think it means, and it doesn't work the way you would like it to work.
Once a unit attempt is finished, that is to say that all the necessary quizzes have been "completed", then the unit attempt is marked as "completed" too, and the students cannot redo quizzes within that unit attempt again.
They can look at a static report showing how they answered questions in quizzes in the completed unit attempts, but they can't then redo quizzes within a unit attempt that is marked as "completed". The closest they can get is to start a new unit attempt from the beginning and work their way through the quizzes again.
So I think what you would like is the same as what Jim, Glenys and probably many other educators would like, and that is to allow the students to have free access to quizzes in a unit once they have achieved a certain grade on at last one attempt at the unit.
hope that is clear