Is there a faster way to load millions of Sales Orders?

Summary:

We have a requirement to load 30 Million Closed Sales Orders with around 290 Million lines. We are using the FBDI method to load these sales orders by batching them into 50k order chunks. One batch in high performing Pod takes around an hour. We are passing SUBMIT_FLAG as Y so that the SO is submitted as soon as it is loaded. so the ask is

1. Should we try any other alternate method like rest api and fbdi combination (high volumn sales order load)? Did any one try this ?
2. Can we load all the sales orders in draft mode and then submit them later in bulk ? This would atleast ensure that the SOs are loaded and submit process would run in the background.