ffmpeg

If that is a true priority, then you do not want Android TV.

Not even a raspberry pi with lineage os?

You would also be hesitant with youtube and netflix

I know. As I said, I’d be using Jellyfin for the most part.

in my case if I use mkv it starts transcoding and mp4 works fine on every device (desktop, android app, Chromecast, browser)

that surely is the issue. you can convert it to mp4 with ffmpeg: ffmpeg -i input.mkv -c copy output.mp4 If you want to keep subtitles this will probably work: ffmpeg -i input.mkv -map 0 -c copy -c:s mov_text output.mp4

you could double check the codec with mediainfo. also check it is mp4.

orgmode with neovim on PC and orgzly on phone. syncing with syncthing

I use Joplin. It supports syncing with OneDrive, Dropbox and NextCloud. Also supports encryption which is great if you are syncing to onedrive or dropbox.

I do it like this. Manage calibre on my PC, and copy the ‘Calibre Library’ contents to the server through ftp.