This isn’t a Fediverse-specific topic, that I can see. This is something that’s fairly trivial to do - there’s already services that will post to multiple channels (“social media management tools” seems to be best search term). In these ten months you could have written one yourself!

Also, the main point of POSSE seems to be that you control your own domain and space online, and everything else points back to that. That assumes that your main way of communicating is via publishing chunks of content, of course, which is patently not true. It also assumes that everyone’s way more technical than they are - plenty of people call their browser “the internet” and would glaze over the second you started talking about domain names, etc.