imo drop bio or extension english. clearly, u like mx2 and uv shown u can improve and same thing with physics. so if u love 4u and ur improving + awesome scaling...i dont see why u should drop it.
imo u should finish all of it...if u skip the easier questions ur more susceptible to silly mistakes in the actual exam (talking from personal experience for 4u).
you can't compare chemistry, physics and biology in terms of how useful they are, individually, to society. they all work together to make our quality of life better...but without them i think we would still be living....but we'd live with a crappier lifestyle.