In Pokemon, is the 'Attack' action the last action that is performed on your turn?
Typically yes, attacking is the last action you take prior to ending your turn. See the TCG Rulebook, page 9.


Attack. Then, end your turn.


Some turns you may take other actions to end your turn and not attack. But there is no second main phase like in Magic. Once you declare an attack there is no more playing cards unless the attack requires it.