You are asking two different questions here.
Your first question is:
Can you perform a ready action with the condition being that you complete your bonus action, then you do your bonus action and then complete your action from your ready action?
And the answer is that yes, you can do that. However, you don't really get any benefit from that, since any bonus action you could take to trigger the action must be a bonus action you could take anyway. You're just as well off not readying an action, using your bonus action, and then just taking a normal action.
Your second question is:
Is this legal exploit to reverse the action order?
The Shield Master feat says:
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.
Though you could ready an action to trigger off of the "try to shove... with your shield" bonus action, you would never actually get to use that reaction and will have effectively wasted your action.
The important text is "If you take the Attack action on your turn...". The Attack action is distinct from an attack and from a regular action. If you've readied an action, even if your readied action is to take the Attack action, then you still haven't taken the Attack action. Since you don't actually take the Attack action, your bonus action of "shove... with your shield" never takes place during your turn.
In this scenario, you do still get to attack if, after readying that action, an opponent somehow ends up both adjacent and prone to you. However, you will still lose the shield shove bonus action if this happens outside your turn.