When my 'Likes and Replies' plugin is done I'm considering building an integrated feed reader that will allow me to read and respond to items all within the same place.

As I wrote before, a self hosted reader tied to your own blog is a logical step; if we want to encourage responses to be owned by their authors at their own properties then it should be as simple as possible to create them.

Whether this is best achieved by an external feed reader that supports posting or one that is integrated with your blog is probably a matter of choice, ability and convenience.

Most readers already support direct sharing to social networks so it's not much of a leap for them to also cater for blogs (via existing methods like XML-RPC, Micropub or the WordPress REST API) with an interface and experience to facilitate it.

An integrated solution would be even simpler.