Best Practice to do a high volume SO Import to Fusion Order Management

Summary

want to understand the best practice of 10000 plus sales orders imports from legacy

Content

there is a need to import 10000 approx. orders to order management,what is the best practice?

some of the pointers that client is using for 2 sites and now in future they need to load 10000 SOs for other sites.

1)They have EFF Attributes both at Header and Lines

2)I need to import with legacy dates ,so I believe I need to write an extension to populate the override schedule to accept the Legacy dates and system does not call GOP again to recalculate the dates and yet should be scheduled so that Planning can recommend supply