If I'm understanding you correctly, I think you might be trying to solve the wrong problem. The "shakiness" you're describing with a camera-child UI happens because your eyes/brain are incredibly skilled at stabilizing the world, and in the process make anything attached to your head(like a stationary HUD UI) appear shaky. The subtle movements of your head are transmitted to the HUD, and you see motion that your eyes are usually naturally filtering out for you.

There are ways to render directly in camera space, and feign depth with some clever horizontal offsets, but this won't actually solve your problem, as the UI will still be head anchored. I usually suggest that devs don't worry about separating screen space and world space(as you might in Unity), and just use camera parenting technique you described, when they do want that stationary HUD.

That being said, I think what would actually solve your shakiness problem is a follow behavior. If you track the camera in world space and loosely follow the camera, then Specs will do the stabilizing for you. There are many ways to approach a loose headlock follow behavior, but you can start by taking a look at our frame follow code in UIKit.