You should not need that override file at all; if you need to override some of the settings from the packaged service unit file, you need to do it carefully, you cannot just repeat the entire contents of the file 
I suggest removing both of the files that you have under /etc/systemd/system, then run ‘systemctl daemon-reload’, then ‘systemctl enable --now transmission-daemon.service’. That will get the daemon running using the packaged service unit file.