Sending SMS via CLI is here:
- Using usermod to add defaultuser to a new group doesn't work - #5 by Beethoven
- Starting sms with dbus-send stopped working on 4.6
- Sending line breaks in SMS over dbus-send? - #4 by ohnonot
As for timing the sending event, good ole UNIX at should work. Or, if you like systemd things, systemd-run --user something something