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.
This is actually a solved problem, using osxcross[0]. The experience is honestly very smooth, and we don't require any apple proprietary binaries. The only thing apple-proprietary is their SDK (containing the header files for compiling, and tbd files for linking), which can be downloaded from apple's website (at least if you have a developer account), or from various GitHub projects archiving them.