College Essay Guy has useful posts and videos about common supp topics.

This site also collects supp questions over several years so you can see which schools really don’t change them from year to year, and start planning in advance: https://internationalcollegecounselors.com/in-the-essay/

Google is your friend on this topic.

If I was starting now, I would decide - are you thinking about the University of California Schools? If yes, they have a series of essays and start there.

Look at the Penn State Honors College essays. These will have a bunch of themes as well.

The challenge with repurposing the "Why Us" essay - is even if you create a model - the word count changes materially.

Why Major and Why Us are the most common.

I agree with the PP to make a Common App practice account as a parent. Be aware that supplemental essays can sometimes change from year to year. The supplemental essays for next fall will not be certain until August 1.

While it's fine to kick around ideas, even outline, I would not bother writing any supplementary essays until at least summer.

Tell us about:
a time you navigated difference
a community you are a part of
why us

You can look up the essays for any school you want by creating a practice account on the common app website.

Parent of current junior here. Question for those that are finishing up their applications - can you please comment on supplemental essay topics that you saw coming up over and over again? I imagine some questions include - why this school , why this major, tell us how you contribute to or how you are shaped by a community, tell us about a conflict or challenge you encountered and how it impacted you, what are you passionate / joyful about - anything else that seemed to be asked on repeat by different schools ? Thanks !