Personally, I'm able to cover all of Marketing in a short period of time since Business Studies just comes easily to me, hahaha. With science subjects, it does take much longer... especially since you need to understand THEN apply THEN evaluate everything else. Although, I only find that issue with Chemistry and not Biology.
Overall, most of my subjects have been chosen out of interest so they all pretty much, somewhat, play to my strengths and I'm able to understand things quickly. I also just really like learning and having structure so that could play a role, haha.
If you can do it, definitely start Finance! However, keep in mind, don't go too far ahead where you end up forgetting the content and wasting your time. Make sure to continuously revise it and consolidate!!