This is the best I could achieve with my poor knowledge of C. Ideally, we should render footnotes into \footnote{} in the case of CMARK_NODE_FOOTNOTE_REFERENCE, but I can't figure out how I can get the footnote content from the node there (maybe that's just not possible). I'd very much appreciate help here.

Could you give some example markdown input with the intended output (to match pandoc or similar), perhaps I can help.

Sure:

Hello[^1]

[^1]: A footnote.

should generate

Hello\footnote{A footnote.}

Currently I'm generating

Hello\footnotemark[1]

\footnotetext[1]{A footnote.}

which is fine but not ideal.