How can I effectively scan the entire trail of my garden to create an immersive augmented reality (AR) experience with interactive objects

To turn an existing environment or object into a 3D digital scene, you likely want to get into Photogrammetry. All Unreal Engine users get a license to Reality Capture which is the best tool for this. There is also the free phone app RealityScan which does the basics.

For large outdoor areas, drone photogrammetry is the best way forward.