Cache flushed
Regarding my note from earlier today, I've now removed the Breeze cache plugin and replaced it with Surge. I've also modified the cache behaviour, as per Dominik's instructions, to support content negotiation. And I've enabled the Vary-Header in the ActivityPub plugin.
Hopefully this will put an end to my issues with visitors getting served JSON versions of posts, instead of the actual website. If you're reading this and ever experience something like that, please do let me know.