i torrent video files via qbittorrent from my PC onto a truenas folder on my server (my old PC), which is attached to a jellyfin jail, from where i can watch it with every device i have wireguard vpn installed on. boom. private video streaming service.
for ebooks, the same thing happens, but i manually move the files onto my kobo libre 2. i don’t have very many ebooks to manage.
the syncthing idea is great, but i suspect your ebook reader is running android. the libre 2 runs a closed down version of linux i believe.