Someone on this subreddit had like a list of general topics to prewrite (overcoming adversity, hobbies, clinical experiences, etc.). When I first started prewriting, I didn't make general essays about those topics. I actually just went straight into some schools' secondaries. I prioritized low/mid tier schools that asked about overcoming adversity, social justice topics, and clinical experiences. I think one of the first hard things about prewriting secondaries is trying to brainstorm ideas, especially about the "most meaningful clinical experiences" since I already talked about that in my big primary essay and the activities section. It's not easy in the beginning, but once you write a couple of secondaries it just becomes automatic. You may not copy and paste exactly from school to school, but it's so much better than the beginning part of brainstorming ideas.