How to show ffmpeg progress with kdialog?

I have found here here a script that provides a GUI interaction in order to trim/cut media files. I have tried to improve it a bit. Now it involves a file picker to select the file, a timestamp window to set the start and end points, a progress bar during the process, and a confirmation message at the end of the process.

But because I am in KDE/Kubuntu I have also tried to use only kdialog instead of yad or zenity, without much success. I have succeeded in using kdialog for the file picker and the end message, but the timestamp window is created with yad (a zenity fork) and for the progress bar I have used zenity.

This is the script:

#!/bin/bash
    
INPUT=$(kdialog --getopenfilename ~/Videos/ '*.m4a *.ogg *.mp3 *.mp4 *.avi *.aac *.flac *.avi *.mkv *.mp4')

eval $(yad --width=400 --form --field=start --field=end --field=output:SFL "00:00:00" "00:00:00" "${INPUT/%.*}-out.${INPUT##*.}" | awk -F'|' '{printf "START=%s\nEND=%s\nOUTPUT=\"%s\"\n", $1, $2, $3}')
[[ -z $START || -z $END || -z $OUTPUT ]] && exit 1
DIFF=$(($(date +%s --date="$END")-$(date +%s --date="$START")))
OFFSET=""$(($DIFF / 3600)):$(($DIFF / 60 % 60)):$(($DIFF % 60))

ffmpeg -ss "$START" -t "$OFFSET" -i "$INPUT" -c copy "$OUTPUT" | zenity --progress --pulsate --text="Running" --percentage=1 --auto-close --auto-kill

if [ $? -eq 0 ]; then
kdialog --msgbox "Process completed successfully!"
  else
kdialog --msgbox "SOMETHING WENT WRONG!"
fi

Looking into kdialog help, it seems there isn't a way to make a timestamp window with kdialog (but that is another question that I'm not asking here: I want to ask these questions separately).

What I want to ask here is how to create a progressbar dialog with kdialog for the ffmpeg process, given that kdialog --help all says:

--progressbar <text>                 Progress bar dialog, returns a D-Bus
                                       reference for communication

I gather that means the use of qdbus/dbus options like these here:

dbusRef=`kdialog --progressbar "Initializing" 4`
qdbus $dbusRef Set "" value 1
qdbus $dbusRef setLabelText "Thinking really hard"
sleep 2
qdbus $dbusRef Set "" value 2
sleep 2
qdbus $dbusRef setLabelText "Thinking some more"
qdbus $dbusRef Set "" value 3
sleep 2
qdbus $dbusRef Set "" value 4
sleep 2
qdbus $dbusRef close

But I don't know how to adjust/integrate such options with that script so that it relates to the ffmpeg process.

With a line like kdialog --progressbar "Running" I can make kdbus show a "dead" window during the process, which doesn't even close when that's over and nothing more. (No "progress" seems to be represented there.)