I think the big different between quarto and typst is the scope. quarto is a tool for combining prose and computation to generate many different output types (HTML, PDF (via latex), PDF (via typst), PPT, ...) through the power of markdown/pandoc. typst is a typesetting system for turning plain-text markup in to beautiful PDFs.
I think you're much more likely to want to write typst by hand than latex, but out of the box it doesn't provide any tooling for combining writing and computing (if that matters to you).
I haven't really used quarto so take this with a grain of salt, but from what I can see it is much more declarative, you just declare the content of your document and pick a template to show it. It feels simplier, but at the same time what if I need to customize something here and there? Looks like there are extensions that can be programmed, but they are more like second class citizens that you are not suppose to use normally.
You're right that typst is _very good_ at extensions, and likely will always be superior to quarto when it comes to that. The fundamental advantage typst has is that it's a "greenfield" project, and a very well-designed one at that, especially when compared to TeX.
> Looks like there are extensions that can be programmed, but they are more like second class citizens that you are not suppose to use normally.
We take quarto extensibility pretty seriously! "Simple" customization is available without need to program extensions, mostly through metadata configuration and classes and attributes in the document. This covers the basics like CSS, layout, document listings, etc.
For slightly more sophisticated extensions, you can create "filters" that operate directly on the document AST, either using the built-in Lua extension API or reading/writing a JSON representation (these are both built on top of Pandoc's capabilities, which quarto leverages extensively).
For reusable, packageable functionality, the extension system as it exists today is simple but certainly meant to be used "normally". It's how custom formats (the common, concrete use case is to provide different styles for particular academic journals) are defined and used.
