I think it's lacking detail/not being specific enough as well as those general 7 mark qs at the end of the paper. About the subjects, thats awesome! I'm looking at doing a double in science/engineering next year
I think earth is easier than chem for detail, because there are less specific numbers or facts. So I would say it's just a matter of practicing including as much detail as you can and knowing your content well. I don't really have any better tips than that sorry.
For the seven mark qs, it's definitely about quality over quantity. So for the general ones along the lines of "why is an understanding of earth science important to protect Australian environments" (I've seen ones like that a few times) I would choose around 2-3 issues such as salinity, pesticides, the formation of the Australian continent, erosion etc. to cover in detail, instead of just naming lots. Again I think all you can do is practice, and have your questions marked.