@angus this is just a suggestion of a functionality that may be useful.
Earlier I suggested Export topic as markdown for external use, which got a great solution. And after a bunch of search I found that it is also possible to move topics and/or categories between Discourse installations. But this is by no means a straightforward action. Maybe I overlooked other, simpler ways to do this, though. An export/import via the UI, idk?
What would make a lot of sense to me would be that the "Select Posts" feature, would also work for the additional forum sections that "have become part of the community" via ActivityPub federation. There are multiple options, and a start might be to only offer the ability to move a full topic from one forum to the other.
Personally I have regular need to transfer between discourse installations.