The only thing left to solve is to actually build cross platform. You can do with rust cross but you have to build your own docker images on macos first since apple licensing prohibits redistribution of binaries. It would save us so much money in github CI minutes.
We achieved this for Firefox macOS builds years ago. (Greg was there for that work as well. ) You do need to have an SDK accessible, but storing one internally and using it in CI was deemed acceptable. The impact on CI build times was hard to overstate.
We were already building our own clang binaries for various reasons, so it was mostly a matter of making an SDK available and ensuring that the right compiler options were passed in (since running clang on macOS sets a bunch of defaults that you don't get even if you pass -target x86_64-apple-darwin).