Hmm, you should be able to set a custom command for it to run before suspend like this: https://unix.stackexchange.com/a/526290
And apparently, you can tell light-locker to lock immediately with light-locker-command --lock
: https://manpages.debian.org/buster/light-locker/light-locker-command.1.en.html
I guess, you might want to try out, if it works at all, before you start editing files. If it works with this, then I'd expect it work when you put it in into the file, too:
light-locker-command --lock && systemctl suspend