Emby Theater for Linux
https://emby.media/download.html
Auto-Start
Locate the autostart file:
~/.config/lxsession/LXDE-pi/autostart
If LXDE-pi is missing then copy it from:
/etc/xdg/lxsession/
Add this line to the end of the autostart file:
@emby-theater
Can you attach the mpv log? adding log-file=/tmp/mpv.log to ~/.config/mpv/mpv.conf should output a log file