Maxperson
Morkus from Orkus
As the examples of 'time-travel'-like rules show, seeming to time travel at the gaming table is not an issue at all! What matters is that no time travel occurs in the game world.
Actions, bonus actions, reactions, the Attack action, the Dash action, 6-second turns....all these are things the player does; the game mechanics at the gaming table.
But in the game world? There are no such things as 'bonus actions', or any other perception of the 5e game mechanics in play. The creatures in the game can have no idea that they are merely our avatars in a game.
In the game world, there are no such things as 'the Attack action' or 'bonus action shove'. No, in the game world there are just attacks, shoves, ripostes, shield bashes...
There are no 6-second turns in the game world. No 'indivisible Extra Attack actions', just a series of attacks and shield bashes. The world doesn't care in what order those two sword slashes and shield shove occurs in terms of where the shield shove is 'allowed' to be.
Mechanically, when you hit with an attack, you hit also your opponent in the game world and deal your damage. When the target THEN casts Shield in the game world, that hit has to be rewound and turned retroactively into a miss. That's time travel. This hit has already occurred in the game world and is being unwound through time so that it never happens.
And, really, neither do the game mechanics. All the game mechanics require is that those attacks and that shield shove occur on the same turn.
Except the mechanics do care. Since there is no mechanical ability to declare an Attack action, the only way to mechanically know if an Attack action is taken during your turn, is to actually take it. That means that there is no mechanical way to trigger the bonus shove prior to taking the Attack action. The mechanics as written don't care if the shove comes after the first attack or later attacks, though. It can't come before, though, as the Attack action doesn't begin until you are taking your first attack.