as a general statement, if u are ever unsure about anything subjects-wise, keep everything for a couple weeks longer so you have been to the classes more and can make a more informed decision. i personally would recommend 12 units for the hsc bc then you could hypothetically fail an entire 2 unit course and be fine, but i know a lot of people would feel overwhelmed w that much stuff to do so it's really personally dependent.
i also have not done ex2 english so i'll let some other ppl give their thoughts but ive heard that if u do something u enjoy then it doesnt really feel like work bc it's fun!