As Siri shortcuts may be a great accessibility feature, Speech Central has launched initial support for 28 Siri commands almost as soon as the iOS 12 has launched.
You need first to perform the command in the app (like pressing the button that will take you to the next paragraph) and after that you can define a phrase in the system settings to invoke this command using Siri (like 'Read the next paragraph').
As this is the initial support some improvements regarding Siri shortcuts are expected in the next version (currently there is no message if you ask for a command that cannot be performed in the current usage context and using Navigation panel doesn't add new Siri shortcuts, it works only if you assign those commands to player buttons).
Since my last post on this site there were several new versions of the app so many things have improved, maybe of specific interest of this audience is that Speech Central now supports importing of books from the Project Gutenberg (Bookshare support already exists).
As always your feedback is welcome as it has greatly helped to improve the app. For new users, the download link is: https://itunes.apple.com/us/app/speech-central/id1127349155