I imagine I could answer this myself by looking carefully at the history of the golang package, but what happens if you end up doing the mass rebuild with a pre-release version? Would you need to do a second mass rebuild with the final version between the beta and final freezes?
The Beta Freeze is currently scheduled for 2022-08-23, and Go 1.19 final is scheduled for sometime in August, so getting the mass rebuild done with the final version before the beta freeze might be tight or it might be just fine.
The beta mass rebuild allows us to detect problems early and sometimes even escalading to the Go team for an incorrect behaviour. They are generally responsive at that stage.