Create Customer On the Fly during Sales Order Creation?

Summary:

My organization creates 7M customers per year, many of which are guest check out from our website. Will have many peaks with thousands of sales orders created per hour, many of which requiring a new customer creation. Our goal is to not required all customer creation REST API calls or FBDI since this process will not be efficient and will not keep up with our requirement.

What is OM solution for creating customers with the sales order creation processes? Can this be accomplished with API or FBDI solutions?

I have read up on EBS solution for Customer Creation On the Fly from the 'Process Order'.