Placing Immersive (Unbounded Mixed Reality) Origin from a Shared Space Bounded Volume

Hi everyone!
I am working on a short animated clip, starting on a bounded volume on a shared space and transitioning to a Full Space (Mixed Reality) scene. I want t recreate the effect done by the Mindfulness app where the user is tasked to place the petals/flower object as the bounded volume box and once that is set, the app changes scene but the object never cuts nor fades, the entire scene just transitions into a MixedReality (or unbounded app).

Does anyone know how to recreate something like this?
If not, another thing I would like to do is create a UI to manipulate a hologram version of my map for users to place the scene where they feel most comfortable, then commit to that selection and begin the experience. I manage to use the manipulation script to move the scene around, but I want to lock Y and avoid rotation.

I am not sure if the first option is native to Xcode and SwiftUI that it might not be a native option in Unity. Help in either solution will be much appreciated as I feel so close from solving this placement interface. Thanks!

which part do you think is not possible in unity?