@rokejulianlockhart , thank you, this worked indeed:
busctl call org.freedesktop.login1 /org/freedesktop/login1 \
org.freedesktop.login1.Manager SetRebootToFirmwareSetup b true && \
qdbus org.kde.Shutdown /Shutdown org.kde.Shutdown.logoutAndReboot
And yes, you’re right, it exists. I then thought (with no skills in software development) the code behind it could be something like executing the command qdbus org.kde.Shutdown /Shutdown org.kde.Shutdown.logoutAndReboot immediately followed by systemctl reboot --firmware-setup .