The reflectance of Phobos is 0.074, while Deimos is even lower at 0.068, so theoretically, the brightness of these two moons should be comparable to that of comets.
As for changing the height map, I originally used the RSS height map, which was also applied to Earth, Mercury, and Venus, while Mars used the IRIS height map. But I thought it was not good to add too many things made by others., so I switched to the default height map.