I thought I had one step left before the migration and went ahead and purchased Forsaken on Battle.net so my pc license was on par with Xbox. Afterwards I realized I had already completed that last step. Will the expansion license sync with Steam in the background or will there by another sync before October 1?
The migration hasn't happened yet, it was just getting set up and ready to do so. On October 1st, your recently purchased Forsaken license will transfer with the migration. The sync up won't happen until then, so you can purchase everything on Blizzard up until the point of no return and it should all transfer normally.