Discourse patterns as low-lying fruit is good metaphor (if we invert stratal model). Grammar is further out of reach, demanding more semiotic labour to learn and apply to text analysis. Halliday’s view……lexical meaning is located much nearer the surface of language… Grammar is much more hidden from view (Halliday 2008:75).
…the patterns we treat as grammatical are those which are buried much deeper below the level of people's consciousness… Lexical patterns are nearer the surface of consciousness (Halliday 2003:120).
Here Rose misleads by strategically misrepresenting the view of Halliday. The Halliday quotes do not refer to the "lexical" relations of Martin's discourse semantics. For Halliday, the lexical vs grammatical distinction is one of delicacy, not stratification.