Is the ATV 4K displaying the photos in 10-bit color or still just using 8-bit color? I have a lot of screensaver photos that are 16-bit, but the ATV displays them using only 8-bit, which causes a lot of color banding. If it would display them in 10-bit the color banding would go away. There is no excuse for Apple to force 8-bit color for photos from our Photos library on a 10-bit 4K HDR TV. That kind of defeats the purpose of a 4K HDR TV.
BTW, 8-bit displays around 16 million colors, but 10-bit can display 1 billion colors, which is 64 times more colors than 8-bit.