ExecStart=/sbin/suricata -c /etc/suricata/suricata.yaml --pidfile /var/run/suricata.pid $OPTIONS
This is the line in the service file that is being used to execute Suricata. You can uncomment one of the EnvironmentFile lines to set the OPTIONS in an external file like you have to up already, are just modify the ExecStart line to provide the command line you want. You’re most of the way there.