The "Before=shutdown.target" is superfluous, that is a default dependency (as regular services obviously need to be shut down before system shutdown).

"KillMode=none" shouldn't be necessary, unless your command leaves processes running that need to stay running (and in that case you have other problems, since those processes may not actually finish before the system is shut down). Systemd waits for your service to stop before reaching shutdown.target (per the default Before= and Conflicts= dependencies).

Since your command probably needs networking, you also want "After=network.target" to ensure your command runs before network is torn down, as covered in sibling comments.