Well.... I am not so sure about applying directly to phd program. Your GPA is not enough considering you are not from the top 3 Korean institutes. Even for the top-3 institutes that GPA is a bit weak. The promising point though is that your final year GPA is quite good, and you did well in UT Austin. You can try applying to the unis that accept last 2 years GPA. But even considering that, I am very worried about not seeing any, not even elementary proof courses in your profile. The best choice for you in my opinion is to go to top3 econ MAs in Korea, which is what most Koreans do, and take the relevent math courses needed. Elementary proofs and real analysis is a must. Considering even those who top their undergrad programs and got econ RA opportunities from their junior years apply to econ MAs first,I believe it's your best option as well.