I think it would be nice to have additional, optional levels after the Master rank. That way people who are worried they might forget an item even after they had finished it can choose to have it appear in their reviews in the future.
This feature has been requested before and is still on my low priority to-do list for now. I can understand your reasoning/need for it and would like to implement it sometime in the future
As a general workaround for now you could do an advanced search (in the cards management table) for your level 9 cards and demote them to a lower level as you see fit.
Thanks for the suggestion!