In most 3D editors, there is a handle in the middle of the translation gizmo that drags the object relative to the camera.
Observe the square in the middle of this translation gizmo.
When the user drags this handle, the object is dragged along the camera’s XY-plane, retaining its existing Z-distance. It has no obvious alternate behavior for local space mode, so I think its behavior should stay the same in both modes.
Roblox should add this handle.
Here’s a video of the handle’s behavior.