I believe you are looking at some way of resource pooling your AFF. There is a SLO pack in automation store, which provides framework for managing your service levels. I am recommending aggregate mapping feature available with this Pack as one of the possible solution. You can create a new service called "AFF" and map aggregates from specific cluster into this service. Now we provide 2 workflows for creating a volume/LUN using this service level. This is just FYI. Refer document attached with the workflows in this pack to analyze if this can work for you.