@mjtsai ever tried screen savers on Apple TV via Home Sharing your photo library? It does NOT support HEIC photos (!)
Dates back to 2017: https://forums.macrumors.com/threads/home-sharing-photos-with-heif-heic-on-atv4k.2081387/
Reddit threads: https://www.reddit.com/r/appletv/comments/n050ad/macos_home_sharing_to_appletv_skips_all_hevc_and/
You can try using iCloud Photos for screensavers using your own photos on Apple TV instead, but no way to use Shared Photo Library with that (+ other weird limitations around albums etc)
Sad, since Apple photo screensavers are genuinely quite amazing with transitions etc