Imagemagick convert: use Pango to format captions in auto generated slides

I use this code to auto generate slides from a .txt file where I wrote captions this way:

CAPTION 1

CAPTION 2

...

CAPTION N

This is the script I use

#!/bin/bash
i=0
# loop through files
while IFS= read -r p;  do
  # if it's not an empty line
  if ! [ -z "$p"]; then
    # echo line
    echo "$p";
    convert -background none -font Trebuchet-MS -fill white -pointsize 60 -gravity center -draw  "text 0,300 'pango:$p'" slide_template.png slides/slide-$i.png
    i=$((i+1))
  fi;
# pass input
done <$@

slide_template.png is simply an empty (transparent) 1920x1080 png.

I pass my .txt file this way:

$ sh my_script.sh my_file.txt

And it generates my slides in /slides.

Now I'd like to use some format code into my slides, like

MY <b>CAPTION</b> 1

MY <i>CAPTION</i> 2

...

MY CAPTION N

But I can't understand how to use pango in my previous code. I need to reposition my caption line centered, 300 pixels from the bottom.

If I use:

convert -background none -font Trebuchet-MS -fill white -pointsize 60 -gravity center -draw  "text 0,300 '$p'" slide_template.png slides/slide-$i.png

I get:

If I use this line:

convert -background none -font Trebuchet-MS -fill white -pointsize 60 -gravity center pango:"$p" slide_template.png slides/slide-$i.png

I get TWO files (why?), where the first one is correctly parsed but cropped to the text size:

And the second one is the background. Filenames this way are slide-0-0.png and slide-0-1.png

Solved: I need to pipe one image to another.

The first contains the formatted code, the second overlays the piped data onto the background.

#!/bin/bash
i=0
# loop through files
while IFS= read -r p;  do
  # if it's not an empty line
  if ! [ -z "$p"]; then
    convert -background none -font Trebuchet-MS -fill white -pointsize 60 -gravity center -size 1920x300 pango:"$p" png:- | convert slide_template.png png:- -geometry +0+800 -composite  slides/slide-$i.png
    i=$((i+1))
  fi;
# pass input
done <$@