The "unlisted" or "quiet public" post visibility setting on Mastodon is often misunderstood.
An unlisted post DOES NOT show up in search results, trends, hashtag follows or the Local/Federated timelines.
An unlisted post DOES show up in the Home timelines of your followers, including when you make a thread with replies.
If you want to avoid your own long threads cluttering up followers' Home timelines, use a Fediverse server or platform that supports very high character limits.