Great questions!
> Would a change in the documentation be sufficient to cause people who don't read it to start reading it? If so, what would these changes look like? If not, what is going to be most helpful to the people, in terms of profitably using hledger, who don't refer to the docs?
Armchair thinking, ignoring cost: probably more prominent help, tutorials, support links/chat built in to the software, more work on error messages, built in templates/examples...
Marketing activity: regular news/tips posts, mastodon posts, scheduled txt/a/v chats/livestreams, podcasts, videos...
GUIs, which make features more discoverable and which are more motivating for many folks.
More visual appeal generally. (Screenshots, colour, charts..)
Prominent testimonials, compelling success stories and case studies.
... ?