An extension could probably be made to quote the full message when clicking reply, but this might not be the best solution for mobile because users might have just as much difficulty editing out everything they don't want, or they'll leave it in the message which will just make reading more difficult for everyone, particularly on mobile.
If the goal is to always quote the full message, the post-mention without quote already offers a quick way to access the full original message by clicking on it (or hovering on it on desktop) so a full message quote is generally not desirable in the minimalistic Flarum UI.
An extension could also add a reply button next to each paragraph. This shouldn't be too complicated to implement, but I'm not aware of any existing extension that does this.