How to show "live" progress of ffmpeg process with zenity?

I have a script to cut/trim media files - more details here and here.

It includes a line to show the progress of ffmpeg process with zenity.

~$ zenity --help-progress
Usage:
  zenity [OPTION…]

Progress options
  --progress                                        Display progress indication dialog
  --text=TEXT                                       Set the dialog text
  --percentage=PERCENTAGE                           Set initial percentage
  --pulsate                                         Pulsate progress bar
  --auto-close                                      Dismiss the dialog when 100% has been reached
  --auto-kill                                       Kill parent process if Cancel button is pressed
  --no-cancel                                       Hide Cancel button
  --time-remaining                                  Estimate when progress will reach 100%

but just using zenity --progress --percentage=1 shows no real progress, the only useful option seems to be to use --pulsate, which at least shows something is happening with a line that goes back and forth.

ffmpeg -ss "$START" -t "$OFFSET" -i "$INPUT" -c copy "$OUTPUT" | zenity --width=400 --progress --pulsate --text="Running" --percentage=1 --auto-close --auto-kill

Trying to mimic this answer I do get a progress bar, but that is shown after the process has ended and the progress itself has nothing to do with the real process.

$(ffmpeg -ss "$START" -t "$OFFSET" -i "$INPUT" -c copy "$OUTPUT")
(for i in $(seq 0 3 100); do echo "$i"; sleep 0.1; done) | zenity --progress --width=400

When I try to use something like this, and the script is adjusted like this

#!/bin/bash

(

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

echo "5"
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")))

echo "5"
OFFSET=""$(($DIFF / 3600)):$(($DIFF / 60 % 60)):$(($DIFF % 60))
echo "10"
echo "# Running processing task." ; sleep 1
echo "35"
echo "# Running processing task." ; sleep 1
echo "60"
echo "# Running processing task." ; sleep 1
echo "85"
echo "# Running processing task." ; sleep 1
echo "99"
echo "# Running processing task." ; sleep 1

ffmpeg -ss "$START" -t "$OFFSET" -i "$INPUT" -c copy "$OUTPUT"

echo "# All finished." ; sleep 1
echo "100"

I get a progress bar but the progress bar is shown before the actual process takes place.

The question is:

How to have something like in the image above, but synchronized in real time with the actual ffmpeg process?