Pretty cool! I made something similar for myself. My primary motivation back then was to make a tomboy-like system to link the individual todo lists together. I use #hashtags to group “zettel” and individual zettel also know other zettel that refer to them. What I also find very useful are [ ] and [x] which I use as lists to check off (seems you do that allready).

Screenshot (it is not publicly available): https://i.imgur.com/js9S4wa.png

But the source is: https://github.com/entropie/ha2itat/tree/main/plugins/zettel

Ill dig a bit in your source.