There are a lot of IndieWeb readers or Microsub clients that fit the bill pretty well. One of their key features for me is that they support Micropub functionality, so you can post your reply directly in the reader and it will send a copy of it to your own website which can also then send a webmention to the original post.

I’ve done some work to adapt Inoreader to support some more IndieWeb-like functionality which you may find interesting.