There are things in the Further Mathematics course in Graphs and Relations that are not taught in Methods. Granted, you would probably pick it up quite quickly, but don't just assume you will be able to go into the exam and fill in that section without any work. I personally think you should stick with what your school has assigned, unless you are finding Geometry and Trigonometry particularly difficult. You won't have the support of your school if you learn another topic so compared to the rest of the state who was taught Graphs and Relations, you would be missing out on some key preparation. The reason why the success rates are lower for Geometry and Trigonometry is that there are a lot of tricks in this section, compared to the others. It's up to you in the end, but I wouldn't waste my time learning new content this close to the exam because it is a period of refining your current skills, not picking up new ones.