OS ≠ user interface.

Use whatever OS that runs Kodi or some other user interface the best (with privacy also being considered to be best).

The more important question is, are they running mainline Linux, close to mainline Linux (like Raspberry Pi, or outdated much modified unmaintainable vendor and device specific fork (like Android phones do).

No demands, just a personal wishlist:

• Stable release
• Federation
• Private/locked/encrypted albums
• Hiding photos from album from timeline
• Dropping Redis dependency (not open source)
• Better editor
• Some (unofficial) Linux mobile client happening