If you are confident that this command will always work (monitor output names randomly change), then create an executable script with this command, and set it as display-setup-script= value in the [Seats.*] section of /etc/lightdm/lightdm.conf.
Test it well, and be prepared for a black screen, just in case… (switch to TTY and rename the file).
Plasma has done a lot of progress with multi-monitors, so maybe you try the available options in System Settings/Display, before using the above (if lightdm itself is not important). You might also want to ask at the KDE forum.