Thanks & success!
Steps:
- add defaultuser to the sailfish-radio group:
devel-su usermod -a -G sailfish-radio defaultuser
- log out/in. For manual sending over ssh that was enough, but for systemd-run more is required - just reboot, or use root privileges.
- Use systemd-run to run at a certain datetime:
systemd-run --user --on-calendar="2025-02-12 19:37:06" dbus-send --system --print-reply --dest=org.ofono /ril_0 org.ofono.MessageManager.SendMessage string:"+9876543210" string:"The message"
The crucial part is that I can close the terminal & ssh connection after issuing that command.
The links above include a different dbus command (involving the messaging app) that does not work anymore.
Sending directly via ofono means that the message does not show up in your messaging app, or any other logs afaics.
This could easily be pressed into a simple app (or, knowing me, a simple shell script).