Just get a range of topics you know really well, and ones you know you'll be able to use for a range of different prompts. Categorizing them is fine, but it'll probably be a lot easier picking a few examples you know really well, and then changing them around to fit different prompts. It's a good tip for the exam, as it'll let you take an essay you really like and rework it to fit different questions.
For instance, I'd often use Bloody Sunday in Northern Ireland as a supporting point in my expository pieces. I used it in reference to people seeking revenge in one essay, and then fr another essay focused on how people change in conflict. I also used that same example later on for an essay based on the search for resolution in conflict, and in my exam, I used it on a piece focused on power.
Just pick a few topics you know really well and you know are flexible, and you'll be much better prepared.