I'm not sure why you would need accounts on all those different platforms. Isn't the whole point of posse that you just post it once and then anyone, regardless of platform, can see it? That's what already happens (with the caveat that some, like lemmy, won't show you certain types of posts, like notes).

And people following you on one platform but not another sounds like more of a desire for multiple identities, each one a fragment of your actual identity. That's another concept, that stuff like socialhub try to implement.