I don't do app development, is targeting Apple Silicon difficult? Don't know what the build process is, but so many apps don't support it I imagine its not a simple case of running a different compiler.
For GPU accelerated applications, like Unreal Engine, you need to reimplement all the rendering logic in Apple's Metal graphics API. That's no small feat for a big engine like Unreal.
Not an option for Unreal for obvious reasons, but it’s an option when you don’t need compute shaders. And believe it or not, the latest techniques won’t get you much further than GL 2 in terms of visual fidelity.
From a technical perspective Unreal already supports iOS on 64-bit ARM processors, so it should have been a fairly straightforward port.