For me, yes, this mapping to topics and replies is pretty close. But I like your analysis of the user experience.
I’m tempted to say, the situation is a little like foreign travel. When you visit another country, you find they eat different food, at different times; you find their language has idioms which are not word-for-word translations. You find this even in two Anglophone, or two Francophone countries. So for me, there won’t be a precise mapping and it will be a battle to try to create one. However, there might be tweaks and nudges, and there might be a useful onboarding experience. Some people will still struggle, because moving to another “country” is difficult.