Asgorath
Explorer
I think the word Now does a nice job of differentiating those ideas. You declare the action NOW. Or you declare the action for later. In your concept the action must be declared now. So then even if the attack action is a discrete sequential event separate from the attacks it grants then it must immediately precede them and doing anything else between the attack action and the attacks would mean that you didn't actually follow through with your declaration and so you didn't actually take the attack action.
I might could get behind that concept. Actions are discrete events that must be declared immediately as you are using them. Then their effects immediately follow. This interpretation would allow any bonus action to be used inbetween extra attacks. It would allow movement to be used after taking the disengage action. *It would allow the shield master shove attack to be used after the first attack but not before it.
I agree with everything here except the final sentence. The trigger for Shield Master’s bonus action is the Attack action, not part of the Attack action. If you split the Attack action and perform other legal activities between attacks such as movement, you must still complete the Attack action before you can do something that is triggered by it.