I found another solution to achieve it under Debian 12 with XFCE4 DE.

`xflock4` is the command I use to lock the session. 

In fact it is a script (discovered by `file $( which xflock4 )`) which contains :

 ...
 # First test for the command set in the session's xfconf channel
 LOCK_CMD=$(xfconf-query -c xfce4-session -p /general/LockCommand)
 if [ -n "$LOCK_CMD" ]; then
 /bin/sh -c "$LOCK_CMD" && exit
 exit_code=$?
 >&2 printf "'%s' exited with status %d\n" "$LOCK_CMD" $exit_code
 exit $exit_code
 fi
 ...

I saw that the `/bin/sh -c "$LOCK_CMD"` is executed only if the `LockCommand` property is not empty.

I tried :

 user@host:~$ xfconf-query -c xfce4-session -p /general/LockCommand -lv
 /general/LockCommand

... and discovered its value is empty in my case then it executes the remaining code which basically tests all locker's applications to apply a lock.

Then I tried to add a value to test it :

 user@host:~$ xfconf-query -c xfce4-session -p /general/LockCommand \
 --set "touch \$HOME/Desktop/touched"
 user@host:~$ xfconf-query -c xfce4-session -p /general/LockCommand -lv
 /general/LockCommand touch $HOME/Desktop/touched

... and tried to lock my session then the touched file is created on my desktop.

---

Then I elaborated the `wait_for_unlock.sh` script :

	#!/bin/bash

	# Script to wait the unlock session (make this script executable).

 # Usage :
 # ./wait_for_unlock.sh /the/script/to/execute/after/unlocking

	# Note : `loginctl list-sessions --no-legend | grep -c lightdm`
	# is my solution to determine when the session is locked ;
	# it was found by tries and is probably not the better solution
 # -
	# Observed :
	# - unlocked when lightdm output is missing
	# - locked when lightdm output is present

	# Here the session is unlocked

	# Lock before continue (here the command may vary)
	light-locker-command --lock

	# Here the session is locked

	while [ 1 ] ; do 
	 is_locked=$( 
		loginctl list-sessions --no-legend \
		| grep -c lightdm 
	 )
	 if [[ $is_locked -eq 0 ]] ; then
		 break
	 fi
	 sleep 0.1s
	done

	# Here the session is unlocked again

 # Execute after unlocking
 eval "$1"

Note : the delay of sleep can be increased to let the system more often idled but not exaggerated because we possibly will wait this maximum time to get the control after unlocking

Then we had to replace the current value of `LockCommand` property, and in my case :

 xfconf-query -c xfce4-session -p /general/LockCommand \
	 --set "wait_for_unlock.sh execute_on_unlock.sh"

And in the `execute_on_unlock.sh` script I wrote all I want to do when the session is unlocked (reminder script must be executable).

Note : to undo this modification use `xfconf-query -c xfce4-session -p /general/LockCommand -r`

The word of end : I discovered that the `--set` parameter of `xfconf-query` must not embed inner quotes in any case nor multiple command (i.e. `CMD1 ; CMD2`) because the `if` statement will considers an empty string then apart modifying the `xflock4` script (see above) which will be replaced in the next upgrade we must use one script with different not quoted delimited parameters.