No.

You can't take a bonus action during another action, thus, you cannot take the shield push's push bonus action until after you've used your attack action.

To address the possibility of taking it before you take the attack action, the power seems to specify that it happens because you take the attack action. If somehow, the shield push invalidates your ability to take the attack action (like, perhaps, you push the only available enemy off a cliff with the push), you've invalidated the ability to take the shield push action in the first place.

(Organized by book, then by non-bashing to bashing, then in an arbitrary order. A short description of the feats' benefits are appreciated but not required, links likewise.)

From the Core Rulebook:

• Shield Proficiency is obvious in the case of characters who aren't already trained in shield-use.
• Shield Focus gives a flat bonus to shield AC.
• Greater Shield Focus gives another flat bonus to shield AC.
• Improved Shield Bash permits the use of shield AC even after performing a shield bash.
• Shield Slam permits a free bull rush maneuver after a successful shield bash.
• Shield Master reduces two-weapon fighting penalties when using a shield as a weapon.

From the Advanced Player's Guide:

• Covering Defense gives adjacent allies a cover bonus to AC when you use the Total Defense action.
• Missile Shield permits a shield-user to occasionally negate a ranged attack against them.
• Ray Shield lets a shield user deflect a ranged touch attack, although the shield can take damage.
• Saving Shield permits a shield-user to grant an ally a bonus to AC.
• Shield Wall is a teamwork feat that allows allies with shields to boost each other's AC.
• Duck and Cover is a teamwork feat that gives a cover AC bonus against ranged attacks if your teammate has a shield, plus some reflex save options.
• Shielded Caster is a teamwork feat that gives you a bonus to concentration checks, increasing if your ally has a shield.
• Shield Specialization gives you better AC against critical hit confirmation rolls, and applies your shield bonus to your CMD with your chosen shield.
• Greater Shield Specialization gives you even more AC against critical hit confirmation rolls, and lets you negate 1 crit per day.
• Mounted Shield adds your shield bonus to your mounts AC.
• Bashing Finish gives a free shield bash after getting a crit with a weapon.

From Ultimate Combat:

• Fortified Armor Training isn't totally shield focused, but lets you break armor or shield instead of taking a critical hit

From Ultimate Magic:

• Channeled Shield Wall takes a use of Channel Energy to give a deflection bonus to AC to you and adjacent allied while using a shield.