Several transcribers I know have this series at different grade levels. I would suggest that you ask the agency how they want this handled. All of the options you suggested are correct; the trick here is to determine which is the main title and which is the series title and that may depend on how the agency has determined that. Ask them and please post the answer here.
I am currently mentoring some transcribers who have this series at Grade 7 and they have chosen your second option.