One place in the lemmyverse that’s been able to buck US-defaultism a bit is the news mega thread on Hexbear. That was done by seeking out and elevating news and voices that are explicitly not in the US (and the Global North, more broadly). But even then, you need both people who are actively seeking non-US perspectives AND people who have those perspectives and are willing to invest their time. I’m not sure how to replicate this more broadly in the lemmyverse, but it’s worth noting where it’s worked and what was done.