Randomly compose images by layering pngs

I am trying to use Imagemagick v7 to batch create images by randomly combining layers which are transparent pngs, all at the same size.

I am a total newbie so I copied the code I found here https://stackoverflow.com/a/27621140/17835805

#!/bin/bash

# Number of output files - edit freely :-)
NFILES=10

# Build arrays of filenames in each layer, assume directories are "Layer0", "Layer1" etc
IFS=$'\n' L0files=($(find "Layer 0" -name "*.png"))
IFS=$'\n' L1files=($(find "Layer 1" -name "*.png"))
IFS=$'\n' L2files=($(find "Layer 2" -name "*.png"))
IFS=$'\n' L3files=($(find "Layer 3" -name "*.png"))

# Produce NFILES output files
for i in `seq 1 $NFILES`; do

   # Choose random index into each array of filenames
   index0=$( jot -r 1  0 $((${#L0files[@]} - 1)) )
   index1=$( jot -r 1  0 $((${#L1files[@]} - 1)) )
   index2=$( jot -r 1  0 $((${#L2files[@]} - 1)) )
   index3=$( jot -r 1  0 $((${#L3files[@]} - 1)) )

   # Pick up files as specified by the random index
   f0=${L0files[index0]}
   f1=${L1files[index1]}
   f2=${L2files[index2]}
   f3=${L3files[index3]}

   # Generate output filename, "output-nnn.png" 
   # ... where nnn starts at 0 and goes up till no clash
   i=0
   while :; do
      out="output-$i.png"
      [ ! -f "$out" ] && break
      ((i++))
   done

   echo $f0, $f1, $f2, $f3 "=> $out"
   convert "$f0" "$f1" -composite "$f2" -composite "$f3" -composite "$out"
done

I managed to make it randomly pick my layers but:

1. no files are created in the output folder
   
2. no matter what I put for NFILES, it always stops at the first one ("output-o.png")

Would be great if I could also make sure that, once a layer has been randomly picked and used, it is not picked again. Perhaps deleting the file with ephemeral?

Many thanks