I'm sorry I didn't quite understand your main question either. Answering your second question, what is the easiest science for you highly correlates with how good you are at maths. Many people consider the order of increasing difficulty to be (as pointed out by Yardlyeverton) Design tech, Biology, Chemistry, Physics; due to the increasing amount of mathematical content in these sciences. If you're really good at maths it is likely that you will rank the difficulty of these subjects in a reverse order. Well, I don't really have an opinion about design tech since we didn't have it in our school, but for me, chemistry required more work than physics, and I'm glad I didn't have biology because it seemed like an insane amount of work.
I also agree that there is more overlap between different sciences than there is between humanities, especially with certain options, but I wouldn't base my choice on that. What I think is the most important is that you like the subject the most (or dislike it the least).