ilmari wrote:As far as I understand, these Heian works use few sino-japanese words - but please correct me if I'm wrong.
In a way this is accurate, but from what I've read this doesn't apply just to kana-only works. Heian era Japanese as a whole apparently used far fewer Sino-Japanese words, even in Japanese poetry written by poets who knew the Chinese classics and could compose Chinese poetry of their own. According to a Classical Japanese textbook I read years ago the major exception to this were Buddhist texts - perhaps owing to the nature of the original texts and the common approaches to translating them. IIRC the mass spread of Sino-Japanese vocabulary into everyday speech didn't start until the Edo era, perhaps owing to the greater spread of literacy among commoners.