This has already been answered. For a national sales tax, the agencies to collect it already exist, and have been collecting state sales taxes for a long time. They are the various state Departments of Revenue. There’s no need for a separate federal agency to duplicate their efforts.
WAshington wouldn't even know how much you made- wouldn't know where you worked - wouldn't have any means of control over you. You would simply pay a National SALES tax, collected at the same time of purchase as your state sales taxes.
Super simple.
a 'flat tax' or any other kind that maintains the IRS will simply morph back into what we have now. After all, the income tax, originally, was minuscule and 'temporary.' It was the camel nose in the tent - as would be the flat tax.
The only thing that can stop it - is NO IRS.