Exceeept, weren't you just arguing that Shield Master couldn't put until after all attacks were made because "when you take the Attack action" meant you had to complete the Attack action?
Actually let's be clear.
I **do not** in my games require the indivisible action ruling. Have said that several times.
So, **in my games** you can make one attack of your attack action, then use the SM bonus action shove anytime after that, even if you have more of your attack action to finish.
That is different from a discussion about the official rules and rulings.
As for the Indivisible action, there is a difference between the SM BA and the TWF that was in the text you quoted.
SM BA
"If you take the Attack action on your turn, you can use a bonus action to try to shove a creature within 5 feet of you with your shield."
TWF
"When you take the Attack action and attack with a light melee weapon..."
The TWF establishes the attack made as part of the Attack action as also part of the trigger specifically. The SM requires the Attack Action taken.
So it seems the two rules establish a difference, one calling out the individual attack in the trigger while the other triggers on the whole attack action.
(Typically, the SM debates hinges on whether you can ignore the attack part of the Attsck action and get your shove anyway... intent bring sll that matters... to some... but that's another story.)
If SM had the same "attack action and attack..." wording as TWF it would be great... establishing that one attack is required before the special shove and like TWF avoiding any issues of having to finish the attack action. (Pretty much, that's how I use it... putting it squarely behind the wet dream of the shove firsters and the overly odd indivisible action of JEC rulings.)