suddenly a post that was crafted for a local community, with users that have a shared culture and background, becomes exposed to a random audience including trolls and bullies

That is a good point, that I've not seen raised elsewhere. Though to an extent, it is already true when you consider the size of aussie.zone compared to lemmy.world for example. Threads will of course be orders of magnitude worse.

Besides the human impact of the negative interactions, the technical impact on the server of providing content for umpteen million Threads users is also a non-trivial risk. This alone is making me think defederation is the better option until Meta have a) released details on how they intend to not overload existing instances, and b) as @david@aussie.zone said "Meta can prove they won’t hurt fediverse".

Having said that, when Threads announce they'll be federating I'll put up some sort of poll to solicit feedback from the wider aussie.zone audience. I don't want to be making such a major change without soliciting feedback in advance.