I want to improve my exposition and writing skills, but whenever I think "what do I know that I can explain to people that isn't explained well elsewhere?" not much comes to mind
If improving your skills is your main goal, you should just write, regardless of whether better explanations already exist elsewhere. Actually, such explanations already existing could even be an advantage, as it provides you with feedback: after writing your own, you can look up existing ones and compare what you did better and what you did worse.
I see what you are saying, but I would be more motivated if it felt like I was doing useful work, and I don't really know what to write about. So I kind of am looking for inspiration/motivation and ideas.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.