Try to plug only Samsung via USB C and unplug the TV. See if it changes the number of available retina resolutions, there should be HiDPI 2560x1440 available at least on any 4K monitor.Great--I'll explore that---it probably is cable-related--but that's frustrating because the M8 is advertised as a USB-C monitor at 3840 x 2160. I'll post my findings here--thank you all for any help!
If this works, you might need to use Samsung as main monitor and the TV as extended monitor.