So, this is definitely not an ideal solution, but until I get a proper solution (e.g. using systemd) to work, I have settled for creating a script (see below) for locking the screen and then suspending, which I mapped to keyboard shortcuts (Super+S for suspend, Super+A for hybrid-sleep, Super+W for suspend-then-hibernate) for quick access.
It's better than nothing, but if someone finds an actual solution instead of this workaround, I'd highly appreciate them sharing it.
The mentioned script:
# Locks the screen by calling "xflock4"
# (plus "sleep 1" to ensure that the screen is locked properly)
# and then suspends the system by calling "systemctl suspend".
# Can be given "hybrid-sleep" or "suspend-then-hibernate" as first argument
# to execute the according action after locking the screen instead of simply suspending.
if [[ $# -ge 1 ]]
case $1 in