epithet
Explorer
Even if you accept that an Action occupies the position of a discrete point in time during your turn (rather than simply taking place "on your turn" generally,) there is nothing other than Crawford's dubious recent additions to his Sage Advice that would equate "take" with "finish." Even if you have the Extra Attack feature, you have absolutely and definitively taken the Attack Action once you have made a single melee or ranged weapon attack. For one thing, making your extra attack is optional, and perhaps more importantly you cannot (unless you use an Action Surge, Cunning Action, or are effected by a spell like haste,) take a different action on that turn.Why is it not possible?
1) Move
2) Attack (1/2)
3) Move
4) Attack (2/2)
5) Move
6) Shield Master shove
You can place the shove anywhere after 4, because that's the point where your Attack action is complete. The Attack action is unique in that there are rules exceptions that allow the action to be broken into smaller pieces, with things like movement in between. Those pieces must all still be processed before the action can trigger something else like Shield Master's shove. I'm not sure why the concept of "are you done attacking or not" is that hard to figure out? If you are still making attacks from the Attack action, you haven't finished the Attack action yet.
Is there anything you can point to other than the recent statements by Jeremy Crawford that would suggest that "take the Attack Action" necessarily requires completing any and all attacks which it allows? Because as far as I can tell, that is a new and preposterous interpretation that Jeremy produced from his backside only recently, and one which does not bear up under scrutiny. I can see a reasonable interpretation of the rules that requires making an attack before allowing the bonus action shove. I don't share that interpretation, but I certainly acknowledge that it is one that a reasonable and thoughtful person might reach. I cannot agree that interpreting the rule to require a Shield Master to complete all attacks granted by the Attack Action before considering the bonus shove to have been triggered is reasonable.