Typst is a markup-based typesetting system designed to be an alternative both to advanced tools like LaTeX and simpler tools like Word and Google Docs.
Typix is a Nix library that aims to make it easier to use Nix to manage dependencies for Typst projects. Examples of this include fonts, images/icons, and data. A rationale is provided in this blog post.
Defining XDG_CACHE_HOME (or perhaps XDG_DATA_HOME?) in the relevant derivations definitely seems like the way to go, at least on Linux. I’m hesitant to provide official support at this time since Typst packages are experimental, and I don’t want Typix to have breaking changes in the future that make it painful to migrate to newer releases.
For now, I could definitely update the documentation with instructions on how a user could provide Typst packages themselves.