I fixed (nearly) everything!

1. Install update for Backdrop from here: https://www.apkmirror.com/apk/google-inc/backdrop-daydream-android-tv/backdrop-daydream-android-tv-1-4-9-release/backdrop-daydream-android-tv-1-4-9-android-apk-download/
2. Now the google photos screensaver option should be available in the google home app.
3. For me, the screen saver worked properly, except when the TV is running fpor >1h, then only the standard photos are shown and not the google photos anymore. To fix this set a static IP! Now, google photos screensaver should work, also turning off the screen via google asssitant should work.
4. Things that are still broken: Turn on TV with google assistant when TV is off for a prolonged time, screensaver activation time is still reset to "never" every 1-2 days...

Does anyone knows how to fix the remaining problems?