@oliphant @ArtBear I wrote a tool to export then reimport posts when moving between Mastodon instances. It's not perfect (it can't preserve timestamps or threads involving others), but it preserves self-threads, recreating them on the new instance.
You can find more details about it here 👉 https://mastodoncontentmover.github.io
(The site is still a bit bare bones right now, as is the tool, but the plan is to keep adding to it)