I would not try to freeze the mix itself because it may become too wet when thawed to form the balls. I would make the balls, place them in a single layer on a parchment lined tray and freeze until they are frozen solid. Then put them in an airtight container in the freezer until you want to air fry them. To cook from from frozen, I would increase the cooking time to about 15 minutes. Personally, I think you will have a better outcome if you air fry the falafel before freezing because there will be less chance of them becoming too wet and falling apart in the air fryer. Just make sure to let them cool all the way and then freeze them individually on a parchment lined tray before putting them into an airtight container.