Writing within 500 characters
500 characters is room for a short paragraph or a list of four or five points, not an essay. Threads posts that start conversations tend to make one point and end with something people can answer, because replies are what keep a post showing up in feeds.
Put the point in the first line. Long posts are cut off in the feed with a "more" link, and the first line is what most people read before deciding whether to open it.
Moving text between Threads, X and Instagram
Threads sits between X and Instagram in length: 280 on X (weighted), 500 on Threads, 2,200 on Instagram. Text written for Threads usually needs trimming for X and can carry more context on Instagram.
Switch the platform in the counter to see the same text against each limit. Keep in mind that X counts differently: it weighs links as 23 and emoji as 2, so the same post can fit Threads and still be over on X.
Your Threads profile is tied to Instagram: the username is your Instagram username, so there is nothing separate to pick there.