I just realized that the Jetpack app supports group blocks(!)
This means that if I want to publish a thread, I can put each intended "post" in individual group inside a single WP post. That allows me to thread things, but also delineate between the items for threads on social.
It's not quite as good as a native block, but it's definitely a step in the right direction. If only I could register my own block in the Jetpack app...sigh. I wish that was possible, because I could make a block that is such a better experience than what I have now.