what rules/guidelines exist for this community. Am I now right in thinking there aren’t any?

None at the moment, other than removing obvious spam/off-topic posts.

Are posts automatically ‘published’ or do they go through moderation first?

Posts should be visible right away. If you posted something and it seems to be stuck in some sort of purgatory, it may have been flagged as a false positive by one of the sh.itjust.works automod bots. I don't know much about these, so I would recommend contacting an admin in that case.

a quarterly “General Discussion Thread” [...] to gather up all the minor stuff.

It's a good idea. In the early days of this community, I tried setting up monthly discussion threads, but they didn't garner much activity. I think the inconsistency with which pinned threads are federated between instances (and platforms) might be a factor here. We could try it again, though, if there is sufficient interest.

Personally, I think the examples you provided would be perfectly worthy of their own posts! :)