Bummer, the formfactor / specs look okay but it's kind of a dead end if I can't just install & use a vanilla Debian OS or similar.
With all the NAS OS options probably Synology has the best one but even there I don't actually want to get locked into that. I doubt this UGOS software can match Synology's let alone Debian.
If it's any consolation it looks like UGREEN is responding to comments about installing other OSes at their kickstarter page https://www.kickstarter.com/projects/urgreen/ugreen-nasync-next-level-storage-limitless-possibilities/comments
Kodi (https://kodi.tv) does the same thing & more. People think of it as a sort of streamer but its original/main purpose is to manage local media downloads including movie/tv series.