This is an interesting thing, but too complicated and over-broad for the mere-mortal. It should be enough to just provide an RSS/Atom feed. WRT comments, well, I never want them on my website anyways, because my blog is for my content. If anybody's interested enough in responding me, they can write an email or another blog post. If I really want comments on a particular post, I post it to HN and/or Reddit.
Anything other than the pages, the feed and a mailto: link is just unnecessary IMHO. If I want a federated social media, no need for implementing and maintaining al this burden, there's Mastodon.
RSS+ITFFF. END. With it you have all the possibilities, even with the comments; only use the RSS comments with your blog.
My first reaction: RSS?
I use Hootsuite to "announce" content, published on sites that I manage, to Twitter and FB.
I also use RSS to "syndicate" content from Site A to x sites in the network.
IMO the web was made for discovery. I don't see why would people would want to consume what I write on Twitter or Facebook where there is no formatting available (or worse splitted in small messages on Twitter). If these people prefer to stay in their walled gardens, good for them, but I don't want to strip what I write to please them. Why won't a link on these platforms suffice since they have their "cards"?
Also if people want syndication in one place, there's still RSS aggregators which is a technology that works even if everyone like to pretend it doesn't.