If I choose to activate the standard critical effect of dealing one damage card face up, I understand that this happens at the end of the attack action, after the defender has used relevant defense tokens.

What about activating other critical effects like Assault Concussion Missiles, or Overload Pulse? Could those effects be activated before defense tokens are used?

If I choose to activate the standard critical effect of dealing one damage card face up, I understand that this happens at the end of the attack action, after the defender has used relevant defense tokens.

What about activating other critical effects like Assault Concussion Missiles, or Overload Pulse? Could those effects be activated before defense tokens are used?

Nope, all relevant critical effect options are decided upon in step 5 Resolve Damage per the RR page 2.

Page 4 of the RR also makes this clear in the Critical Effects entry.

Unless a card specifically states a different timing of course.