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.
That’s relatively easy to do, with the only questionable/legally gray area being access to the iOS/Mac SDKs. I’m not a lawyer, but it’s my understanding that the Google v Oracle case established that APIs aren’t protected by copyright. So if we can use the headers from e.g. the iOS SDK on Linux wtohout worrying about copyright issues (Apple developer agreements may be an issue though), it should be possible to autogenerate stub libraries for linking purposes.
With that, you can compile and link a binary for iOS or Mac on Linux.
Next, you’ll need to implement the packaging and signing portion. Packaging (generating an app bundle) is easy, but codesigning requires Apple’s codesign utility, which doesn’t work outside of Mac, even with a project like Darling (last I checked, anyways).
But if the OP does implement a working codesign alternative, then that means it should be possible to create an iOS/Mac app that can be distributed on the App Store entirely from Linux with no Xcode, virtual machines, cloud macs, etc.
The only gate left then is the Apple tax/signing certificate you need to buy for $99/year. But that might change with all of the antitrust pressure Apple is facing, including the recent EU law that would force Apple to allow competing app stores.