The only reason to go out of your way to get Step 2 done early is to improve upon a sub-par Step 1 score. Depending on the tier program for which you're aiming, I think you could call that anything below 220-240. While you're right, there are fewer rad onc applicants, I'd be surprised if a low step 1 score doesn't eliminate you from consideration at some schools, or strongly weigh you down at others. That's not to say you should delay step 2 if you did well on step 1. Take it when it make sense for your schedule, and do it early/kill it if you're working with a low step 1.
After that, yeah it's research and rec letters. Letters are huge in this field. Get good ones from famous people, and get them in before the submission date. Yes you can send them later, but avoid.