In my case, I just use one Org file per card. I don't use internal links. I prefer search (ripgrep) and index files, where I list sequences and entry points to topics. This also allows restructuring my Zettelkasten more easily once new ideas emerge, or once I realize my old ideas were wrong.
In other words, I am trying to go one step further by decoupling cards (ideas) from links (relationships).
I do use external links, and in particular DOI links to publications. But I keep these in footnotes. Hence, my zettels look really spartan. It's just plain text plus footnotes.
Aside from Org and Markdown, TiddlyWiki is pretty neat. It's pretty good for transclusion. That can also be achieved in single-file Org Zettelkastens / wikis.