What works best for me is to add a link to the fediverse, linkedin and sometimes HN threads of comments when I publish a post. It's also a very low tech solution :)
At the same time, I am an opportunist — sometimes it is Reddit, sometimes it is ix Xm, and sometimes it is here, the Hacker News. Different posts resonate with different audiences; and I don't want to link to a ghost town.
One issue is that there is no such thing as "the default platform for discussions"—no matter which one you choose, only a small fraction of readers will be interested in participating.
Isn't this what the various linkback mechanisms tried to solve? It seems like the ideal solution: let anyone comment wherever they want, from a tweet to their own follow-up blog post, and connect them all via automated notifications.