The way the game is and was written, you have to take the attack action first(past tense_. In the middle of taking the action and about to take the action are not qualifiers to trigger the feat. They are justifications for those who choose to "interpret" the feat that way, to do something that the game doesn't allow.
Totally not the case, from a simple grammatical perspective. "If you take the Attack action on your turn" is not the same as "after you take the attack action," nor is it the same as "once you have completed the attack action in its entirety," nor is it in any other way past tense. "You take" is present tense, as opposed to "you took" or "you have taken."
Similarly, "you can use a bonus action" is also present tense, and not future tense. It is not "you will be able to" or "you can then take" or anything of the sort.
I will agree that in most cases, an "if a, then b" structure suggests that "a" come first, but certainly not all the time. In this instance, it seems most reasonable to read "a" and "b" as happening at the same time. Take, for example, the wording of the Extra Attack feature, "whenever you take the Attack action on your turn." That's still an "if a then b" logic structure, but the word "whenever" implies concurrence. In fact, that is exactly how I've been reading the feat all along, reading the "If" the same as a "When," so that the Shield Master can choose to have an Extra Extra Attack that can only be used for the Shove and precludes other bonus actions on his turn, but otherwise (especially for timing) works the same as the Extra Attack. Slice, shove, slice... shove, slice, slice... slice, slice, shove... no difference.
This whole concept that the Attack Action is the same as the attack you make "with this action" (note that "with this action" also doesn't really carry timing constraints) is new, and the fact that Jeremy has been beating that drum a lot in the past few weeks doesn't change the fact that it's new. His insistence that declarations don't count opens other issues, as well. Take, for example, the Sanctuary spell. "Until the spell ends, any creature who targets the warded creature with an attack or a harmful spell must first make a Wisdom saving throw." If you fail the saving throw and don't choose another target, then you don't make an attack and therefore under the new Crawford interpretation you have not taken the Attack Action, and you're free to Dash, Dodge, Cast a Spell, etc.