Hi BrokenPoly.
Have you tried setting ‘Mesh Space Rotation Blend’ to True in the settings of your Layered Blend per Bone node? Having the rotations in mesh space should help to preserve some of the rotation that your spine bone is losing when you remove the Root bone from the blend.
In case you’re unaware of it (I was for a long time), something else worth noting is that you can mask out bones that would normally be influenced by a LayeredBlendPerBone by giving them a negative Blend Depth. For example, if your branch filter is a spine bone but you don’t want the left arm (a child of the spine) to be influenced, you could add another Branch Filter for the left arm and set it’s Blend Depth to -1. Might look something like this:
Good luck!