Ah, I tried that before but I tried again and it worked! Setting "video" made the brightness value changeable in KDE (and the keys worked) but no actual change in the display. Setting "native" made it all work fully, with the display reading as 'amdgpu_bl2'. Out of curiosity, what is the actual issue here? Is it just a matter of defaults chosen by Kernel, distro, etc? Or is there something Lenovo should have done to make this work out of the box?