After a little research, I've found a solution, although I'm still open to other (probably better) answers.

Here's a startup script (which can be added to Startup Applications) which will set the maually entered usbkbd_layout variable to the usbkbd device ids found in the xinput -list:

#!/bin/bash

usbkbd=`xinput -list | grep -c "USB Keyboard"`

if [[ "$usbkbd" -gt 0 ]]

then

    usbkbd_ids=`xinput -list | grep "USB Keyboard" | awk -F'=' '{print $2}' | cut -c 1-2`

    usbkbd_layout="tr(f)"

    for ID in $usbkbd_ids

    do

      setxkbmap -device "${ID}" -layout "${usbkbd_layout}"

    done

fi

exit 0

This script is quite useful (and more stable) for scenarios where user starts using the laptop on a desktop setup (with external keyboard, mouse and monitor, etc.), and it can also be run manually whenever the external USB keyboard is plugged in...

==========================================================================

THE BETTER (almost perfect) SOLUTION - found thanks to MinimusHeximus and the respective contributors to the thread he mentioned in his comment below:

I can now just plugin my USB keyboard and automatically have its different (TR-F) keyboard layout applied while still keeping the default keyboard layout (TR-Q) on my laptop!

Here are the files and their contents that make this possible:

/etc/udev/rules.d/00-usb-keyboard.rules

ATTRS{idVendor}=="09da", ATTRS{idProduct}=="0260", OWNER="sadi"

ACTION=="add", RUN+="/home/sadi/.bin/usb-keyboard-in_udev"

ACTION=="remove", RUN+="/home/sadi/.bin/usb-keyboard-out_udev"

/home/sadi/.bin/usb-keyboard-in_udev

#!/bin/bash

/home/sadi/.bin/usb-keyboard-in &

/home/sadi/.bin/usb-keyboard-in

#!/bin/bash

sleep 1

DISPLAY=":0.0"

HOME=/home/sadi/

XAUTHORITY=$HOME/.Xauthority

export DISPLAY XAUTHORITY HOME

usbkbd_id=`xinput -list | grep "USB Keyboard" | awk -F'=' '{print $2}' | cut -c 1-2 | head -1`

usbkbd_layout="tr(f)"

if [ "${usbkbd_id}" ]; then

    gsettings set org.gnome.settings-daemon.plugins.keyboard active false

    sleep 2

    setxkbmap -device "${usbkbd_id}" -layout "${usbkbd_layout}"

fi

/home/sadi/.bin/usb-keyboard-out_udev

#!/bin/bash

/home/sadi/.bin/usb-keyboard-out &

/home/sadi/.bin/usb-keyboard-out

#!/bin/bash

sleep 1

DISPLAY=":0.0"

HOME=/home/sadi/

XAUTHORITY=$HOME/.Xauthority

export DISPLAY XAUTHORITY HOME

gsettings set org.gnome.settings-daemon.plugins.keyboard active true

Notes:

1. Of course all of the four files in your ."bin" folder should have necessary permissions (readable and executable) which maybe
   implemented for example with a Terminal command like chmod - 755
/home/sadi/.bin/usb-keyboard-*


2. Sometimes after the USB keyboard is plugged in it still uses the same (default) keyboard layout, and switches to the specified layout upon the second try (perhaps requiring a little more sleep time somewhere?)


3. The USB keyboard specific layout is not effective in the login screen (when you Log Out).


4. If you use a separate partition for /home, then it might be a better idea to put the four scripts somewhere in the root partition, e.g. /usr/local/bin and modify the contents of all respective files accordingly as sometimes udev may look for those files before your /home partition is mounted and cause problems.

IN ORDER TO ADAPT THIS SETUP TO DIFFERENT REQUIREMENTS:

1. USB keyboard vendor and product ids should be changed as per the output of the command lsusb (For example, my lsusb output have this for my USB Keyboard: Bus 001 Device 006: ID 09da:0260 A4 Tech Co., Ltd)


2. OWNER and all user directory names should be changed from "sadi" to another name


3. The usbkbd_id may require a little adjustment to grab the correct device id (For example, output of the commands xinput -list | grep "USB Keyboard" gives me two lines; ↳ USB Keyboard id=14 [slave keyboard (3)] and ↳ USB Keyboard id=16 [slave keyboard (3)]; which are then filtered by awk using "=" as field delimiter and capturing the second part; then cutting only the first two digits, and then using only the value in the first line)


4. The value for usbkbd_layout may be any other valid choice