One other possibility is use git log --follow to grab the original filepath + commit hash, to base the code on. Imperfect but should reduce code changes vs. on rename.
I really like the simple 4 letter code for shorthand!
It's an interesting problem to think on and also reminds me of resource fork advantages. My two cents is it's worth having a setting for "more permanent" codes at whatever compromise it forces. The idea of very short note codes is really nice, especially if it's "that's the code, it won't change" (I constantly rename and move notes to improve organisation and I expect others do) — I'll be trying out your plugin soon!
The Git idea was to track the original commit of note, assuming the vault is in a repo, and use that original path + git hash as the thing that doesn't change. Log tracking is imperfect, but I'd expect it to work in a vast majority of cases in Obsidian context.