You can implement a simple one without much effort. But doing anything a bit more interesting need to dig into zle, and there is only a few resources available online.
You can write completions declaratively and use TypeScript to generate dynamic suggestions.
Disclosure: I'm one of the founders.
I've though about using tree-sitter DSL to generate zsh completion.
Your project seems to be a better candidate for this. Tree-sitter DSL, with some extra annotations, could be used to generate the specification. With the added bonus of being a format that can provide syntax highlight on text editors (in commands in sh files).
If I not mistaken, tree-sitter also uses TS.
I only gave a quick look at fig (only use HN on mobile). Will have a better look when at the desktop.