In his 1993 book The Lexical Approach, which I just checked out from the library to look for a kind of taxonomy of lexical items but will probably want to hang onto for other reasons, Michael Lewis lays out some categories for lexical items. This is important, because for Lewis "language consists of grammaticalized lexis, not lexicalized grammar." I don't know how hard this actually pushes against linguistic theory; Lewis is pretty strictly focused on pedagogy, though he does claim to be doing applied linguistics (and I think he definitely is); he cites Halliday on occasion, but in terms of his understanding of language as primarily lexical, he cites one article by linguists and one by an ELT specialist.
I am interested in how/whether grammar and lexis are categories that can be called separate in any meaningful way, and would probably like to do a short section on this in the introduction to my data analysis for my first research question, but at the moment I'm less concerned with a detailed taxonomy of 'types of lexical items' than I might have been before. I'm still thinking about this, though.