Hypothes.is allows for a few things:

• The ability to reply to specific portions of a page (similar to the functionality in Medium.com)
• A great/simple UI for quickly writing replies, notes, or even bookmarks relating to particular page as well as tags

They have a fledgling API as well as RSS/Atom feeds which I can pipe directly back into my site to own the data, post it, and send Webmentions if necessary. At present, I’m using a variation of a posting flow that uses IFTTT to get all the data back to my site’s Micropub endpoint: https://boffosocko.com/2020/01/21/using-ifttt-to-syndicate-pesos-content-from-social-services-to-wordpress-using-micropub/ (I’d eventually love Hypothes.is to be a fully featured Micropub client, but one thing at a time.)

Good luck on the explorations! If you have issues/questions hop into the chat.