I'm unfamiliar with SwiftUI, but fwiw, I found the extensions needed for CSS3D to support AR to be surprisingly small.
A few years back, having a custom browser-based VR stack with passthrough AR, I sketched a talk for the BostonVR meetup, to give around April Fools. It would have purported to be an introductory onboarding demo, of newly available CSS3D extensions for AR. With support for placement in realspace, billboards, HUD overlays, integrated multiple displays, position aware and 3D displays. The extension needed was surprisingly minor. The talk would have been basically "introductory CSS positioning, in a slightly enriched context", which I expected to be quite believable, followed by a "surprise! - the making of the demo spike". I don't quite remember why it didn't happen.
> 2(.5)D experiences
Shallow-3D UIs seemingly have a lot of potential, but don't get much discussion.
Meta: I wish HN discussions around AR were of higher quality. It's be clear for years that Apple was pursing this. Being unfamiliar with the Apple ecosystem, I'd have liked to see more discussion of what those pieces and their characteristics might suggest about the future. Or for instance, of whether Apple is doing any CEP complex event processing, or retraction of app state changes, to support input with diverse latencies.