A glossary that links itself
Marking a term by hand beats detecting every occurrence of it, and the reverse direction comes free.
Auto-linking every occurrence of a defined word is the obvious version and it reads as machine-generated by the third paragraph. Marking the occurrence that matters is a small amount of work at exactly the moment you know which one it is.
Once the link is a real reference rather than a string match, the term page can list every post that points at it — and that list is a query, not an index I maintain. The whole thing stays a deep module because nothing outside it knows how the reverse direction is computed.
*[_type == "post" && references($termId)]{ title, slug, publishDate }