Slim down an existing Raspbian install. See https://blog.samat.org/2015/02/05/slimming-an-existing-raspbian-install/

Slim-Raspbian.sh

#!/bin/bash

# GUI-related packages
pkgs="

"

# Edu-related packages
pkgs="$pkgs
idle python3-pygame python-pygame 
debian-reference-en dillo x2x
scratch nuscratch
timidity
smartsim penguinspuzzle
pistore
minecraft-pi python-minecraftpi
wolfram-engine
libreoffice*
"
# Because of of https://github.com/RPi-Distro/raspberrypi-ui-mods/issues/2 (thanks @robertely)
echo apt-get -y remove raspberrypi-ui-mods

# Remove packages
for i in $pkgs; do
	echo apt-get -y remove --purge $i
done

# Remove automatically installed dependency packages
echo apt-get -y autoremove

# Remove all packages marked rc (thanks @symm)
dpkg --list |grep "^rc" | cut -d " " -f 3 | xargs dpkg --purge