Honestly, both GCSE and IGCSE are quite easy, especially English Literature, Math and Geography.
Subjects I would focus on are - Economics, History, Biology, Chemistry and Physics. For the rest of them you should be fine.
If you want to do Psychology in the future (in university) it could be beneficial taking a GCSE course because you'll know if you would really like to study it or not in IB/A Levels, but it is not essential for university.
IB Psychology is incredibly difficult - every single student in my school struggles with this subject. So perhaps A-level is easier? But if you want to study Psychology in university, then IB Psychology will definitely help a lot. IB is good preparation for university or other HE courses.