You can not perform the bonus action attack prior to the Attack action (which you attack with a one-handed light weapon), as the bonus action attack depends on you performing this action.
Explanation of Bonus Actions: If a function specifies that you can run X as a bonus action when you run Y, you must execute Y before you can execute X. For Shield Master, this means that the bonus action must occur after the attack action. You decide when it happens afterwards.
That contradicts an earlier decision from him via Twitterin which he said: "As with most bonus promotions, you choose the timing so that the Shield Master Shove can be done before or after the Attack action." Crawford acknowledges this change here:
In 2017, I changed the bonus schedule schedule because the old rule was illogical. The original decision did not take into account the fact that X, which relies on Y, is a form of timing. The new judgment corrects this oversight.
The old bonus action timing rule did not satisfy questions about this timing. Instead, the illogical ruling triggered questions, and it even unintentionally led some fans to assume that our wording "whether" or "when" had very precise meanings for bonus promotions. They do not do it.
As you quote, the two-weapon fighting rule is:
If you perform the action of attack and attack with a light melee weapon that you hold in one handYou can use a bonus action to attack with another light melee weapon you hold in the other hand. You will not add your ability modifier to the damage of the bonus attack unless this modifier is negative.
The prerequisite for the bonus action attack is that you use the Attack action and attack with a suitable weapon. For this reason, you can not use your bonus action attack until you have performed the action that qualifies you.