Turns out macOS 15 finally provides some tools to create a custom FormStyle. I've published an example project here: CustomFormStyle for macOS (on GitHub) This isn't a drop-in replacement and has some drawbacks, but it may be helpful increating your own FormStyle.

This is a new behavior in macOS 15. GroupedFormStyle now limits its content width to 600. In macOS 14 and earlier, this wasn't the case. Unfortunately, I also haven't found a workaround yet and creating a custom FormStyle that mimics .grouped seems almost impossible.