Grammar Pedantry Warning!!
"You take" is not "present" tense, but, rather present simple and is used for actions that are repeated over time. "If you take" is conditional tense and typically follows that the conditions must be met before the second part of the sentence occurs.
Such as, "If it rains, I will take an umbrella" would be most common. If/can clauses are also typically read this way. "If I have a driver's license, I can legally drive a car" would be a good example. Without the conditional, the result is not possible.
So, a grammatical reading of "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." would read that the Attack action must be taken before you can take your bonus action. The Attack action cannot be broken up - additional attacks are not additional actions. They are simply part of the same action.
(Barring, of course, specific exceptions like moving)
The RAW reading of this, coupled with an actual grammatical reading would support Crawford's interpretation. Conditionals are not read as suggestions nor are they read as having more conditions than what is stated. If/then conditionals are what they are. You take the Attack Action, then you can take the bonus action from Shield Master because the condition for taking the bonus action is that you take the Attack Action.
Now, granted, I won't be changing how we do it in our game. But, from a RAW reading, yes, I can see why this would be the RAW interpretation. For you to take the bonus action in the middle of the Attack Action requires reinterpreting the Attack Action to mean that gaining multiple attacks creates multiple Attack Actions, which can be interrupted. However, RAW doesn't support this. Attack actions are discrete - regardless of how many attacks you actually make.
Pedantic? I?
I like your approach, but I disagree with your conclusion.
Some actions are basically instantaneous. When you switch the light on, you either have or you haven't.
But some actions are continuous, and if you
start to do something but are still in the middle of doing it then you can't claim that you're not.
As for the conditional:-
"
If you take a law degree,
then you may use the law library".
This is an 'if/then' statement, where the action (take a law degree) is continuous for several years! The purpose of the law library is to help law students pass their law degree, but those students who are not taking a law degree are not allowed to access the law library.
Sure, the conditional must be satisfied in order to get the result. But if it were the case that that the conditional must in all cases be started, gone through, and completed
before the result, then how would it work in the above example?
'If you take the law degree', with that understanding, would mean that you cannot access the law library until
after you have completed your degree!
That obviously (and I hope it is obvious!) would be absurd! The conditional does not have to be
completed.
In fact, in some cases it might not even have to begin!
"
If you take a law degree,
then you can take a room in the law dormitory".
You are allowed to take a room in the dormitory before you even attend your first lecture, before your course actually starts. Just saying you will, in the immediate future, take a law degree is enough.
"You cannot practice law until you have passed the bar exam".
Here, it is not enough that you
will pass the bar, or that you are in the middle of passing the bar, you must have
completed passing the bar in order to practice law! Note that 'passing the bar' is really an instantaneous action rather than an ongoing one.
So, pedandically parsing Shield Master:-
"
If you take the Attack action on your turn, (
then) you can use a bonus action to try to shove a creature..."
While executing a single attack is an instantaneous action with just a 'before' and 'after', the Attack action is an ongoing action that could last from the beginning of your turn to the end if your Attack action lets you take more than one attack.
So, as with the law student example, merely being an if/then conditional certainly does
not imply that the continuous action must be
completed before you can shield bash, and I would even argue that you don't even have to have executed the first attack yet, although the second one is debatable.