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.
a.ley
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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.