You might be misunderstanding how object radius is calculated, which is based on the Static Meshes scale set within the Static Mesh Editor, and not the scale set within the Foliage Paint. These are not linked on purpose as they operate differently at their core functionality.
Object Radius
The typical workflow is to create multiple meshes to be computed as your levels of detail based on screen size. You don’t need to create multiple materials, but only one master material, instance that material , and have that applied to the LOD mesh based on the screen size/scale which you can set up within the Static Mesh editor.
Uncheck the Auto Compute LOD Distance, so you can manually set at what screen size your material instance is applied.
LOD Settings
I really suggest looking into Mesh Distance Fields in conjunction with foliage as it improves performance greatly. We used this feature in our recent Open World GDC demo to create a visually stunning and asset heavy scene.
Mesh Distance Fields
Cheers,