A RECENT DISCOVERY IN THIS THREAD BY THE ORIGINAL POSTER:
Quick review... I updated my Apple TV 4K (Ethernet edition) to tvOS 26 and my Apple TV 4K would not complete the syncing with iCloud Photos so I can use the Screensaver with one of my Photo Albums. I even performed the subsequent updates to Apple TV.
This morning, I moved the Apple TV 4K device to my office where I could plug it in directly to Ethernet. Immediately the device finished syncing and now everything works fine. However, another Apple TV 4K in the master bedroom (non ethernet) can't complete its' update syncing with iCloud Photos either.
Item of Note: I am an instructor at a media arts college and I have super fast internet service because I do a lot of Zoom sessions where I demo motion graphics and such. So the issue is not the speed of my internet when it comes to using Wi-Fi only.
So, I've pinpointed the issue to an ethernet/wi-fi issue for my home. Is it the type of Router I'm using? Could be... not sure. Is there a sneak setting somewhere that I heretofore have not discovered that needs tweaking... maybe.
I still think it's an Apple thing because why would these devices all update to iCloud Photos before but are now unable to since updating to tvOS 26.
Anyway... there's the latest in this thread and unfortunate issue.