This isn’t probably the best way do do this, but here’s what I found:
First, re-apply current settings as the configuration file may have been altered manually
qdbus org.freedesktop.ScreenSaver /org/freedesktop/ScreenSaver org.kde.screensaver.configure
Delay before locking (locking is enabled if different than 0)
kreadconfig6 --file kscreenlockerrc --group 'Daemon' --key Timeout --default 5
Lock after resume from suspend should be enabled
kreadconfig6 --file kscreenlockerrc --group 'Daemon' --key LockOnResume --default true
Should require password after locking, plasma now allows unlocking without password
kreadconfig6 --file kscreenlockerrc --group 'Daemon' --key RequirePassword --default true
Delay before password required after locking
kreadconfig6 --file kscreenlockerrc --group 'Daemon' --key LockGrace --default 5
Found these keys after changing the Screen Locking settings while looking at ~/.config/ and then watching the changes on ~/.config/kscreenlockerrc
Related: command line - How do I programmatically disable the KDE screen locker? - Ask Ubuntu