Australian here. Sorry that's not a thing. Maybe if you gestured the thumbs up in a particular way? (Usually moving your whole arm up and of towards your shoulder? Honestly hard to explain in text). But that's kind of a whole new gesture, not a thumbs up.
Don't be afraid of doing the thumbs up here, we all do it and know what it means.
Ahh there you go, maybe hitchhiking culture was different down here. Although I don't think I've ever seen someone attempting to hitch-hike in my life tbh. Just not something people do anymore I guess. (Maybe it happens in more rural areas).
You're not wrong about your culture spreading though. Halloween wasn't even considered a thing when I was growing up. Now... depending on where you live you get kids attempting it. But it's still a minority. Many grumpy home-owners saying "this isn't America!" still exist.
'Dag' is certainly some aussie slang. Although I've never heard it used like that, I spose that might be its origin (sheep shearers are kind of a historical working class icon here). These days it's probably be more synonymous with "dork", or wearing some unfashionable clothing.