I want to develop Apps for the Apple XR Headset when it’s officially announce in 2023. What development path would you recommend? Learn SwiftUI, ARKit, RealityKit or wait until the SDK for the Headset is released?
Welcome to the community! Where do you hear there’s going to be a headset in 2023?? I haven’t seen that yet! 
I’d recommend ARKit and RealityKit frameworks, from Apple’s WWDC videos
You’ll need to know Swift the actual programming, that those frameworks use
I’m not sure exactly about SwiftUI vs UIKit with these, cause generally you’re creating models / objects, and I’m not entirely sure if that uses a UI framework or if it relies more on SpriteKit / SceneKit
It is pretty save to assume, that both ARKit/RealityKit and SwiftUI will be at least good to know for XR projects. It’s possible that a new 3D UI framework will be released alongside new hardware, but the principles are probably pretty similar.
I would recommend getting comfortable with SwiftUI first, as it is ultimately what everything else will depend on.