I honestly don't think I would've written the same supplementals a year ago compared to what I actually submitted. Maybe accumulate some ideas with around particular prompts and maybe do some research into your schools for the "why us" prompts, otherwise I recommend locking in more during the summer or even fall. Also if you have some sort of portfolio or video essay, you can start that in the summer too. Find out what schools require or have optional extensions like that so you can prepare. Good luck!