@dan (tl;dr version: ActivityPub is a protocol that passes around JSON-LD objects. Any project doing things "the right way" has an export tool, and that export tool gives you a flat JSON file, and an import tool that accepts it.
As for "does anything actually do things the right way", I don't know about Pleroma's import/export facilities, but Mastodon provides at least the export tools at the account level. It exports accounts as a JSON-LD object containing the entire ActivityPub actor. It's one of the very few things Mastodon didn't get all Not-Invented-Here about.
@firstname.lastname@example.org that makes sense and I've found those objects in my database. It's the import part I'm missing - I can't just post my entire notes history to the new server's outbox or it'll send them on to all the original recipients
"pissed everyone off in literally record time" - Recommended by 10 out of 10 people who, for some sad reason, have a dedicated column up to watch #fediblock.