Antlr sounds excessive for either of those. Use an ordinary xml library for docx (if there's not already one for docx) and something simple for typst.

ANTLR is for writing parsers. You don't need a new custom parser, just use an existing XML parser.

I'm not sure what the best approach would be, but for reading docx you might be better off using something like Apache POI. Docx may be XML, but it's imo absolute abuse of XML. POI shields you a little bit from all the nonsense happening in docx. I could see ANTLR working for Typst since there's probably not another interface for it.

I don't think it'll support it, but you could also check if this can be done with pandoc.

I don't know anything about Typst, but I do know that .docx files are really just a zip file containing a folder structure with a bunch of xml (and a few other) files. I've written a few find/replace docx scripts in bash utilizing this information.

Since the source is XML XSLT may work to transform it.