If you had 6 interviews, then it means your app had no red flags and you're a pretty competitive applicant. But for all of them to be waitlists means you need to put some serious muscle into interview practice. If any of your schools that you interviewed at gives feedback, definitely reach out and see if they could tell you where some of your interview weaknesses are. Practice, practice, and practice is key! IDK if you had MMIs or traditional interviews, but practicing both is important. Finding a mentor to practice with or maybe current medical students may help.
I suppose sending LOIs won't hurt though. If none of your WLs turn to As (I'm knocking on wood for you), then rewriting your essays and getting more clinical experience is good, but definitely working on interview skills is a must.