How can I change whatever program KDE5 uses for screen locking to slock? AFAICS there is no such option in the system settings.
There is no built-in way in KDE5. You'll have to disable the KDE screen locker, then manually run something to trigger
slock. The method is similar to my answer here, but I'll summarise below.
You can use something like
xss-lock to automatically trigger the screensaver. You can start this automatically on login, by creating the following file at
[Desktop Entry] Exec=xss-lock -- /path/to/slock & Type=Application Terminal=false Name=slock_trigger
To change the timeout value, use
$ xset s TIME_IN_SECONDS TIME_IN_SECONDS
TIME_IN_SECONDS with the number of seconds of idleness before you want it to activate. I don't entirely understand why you need to enter it twice, but that's what worked for me.