Here's the problem:
chromium-browser <url> --start-fullscreen
That command will run forever until the user closes the window.
Same reason why running this command in the terminal will not return you to the prompt (
pi@raspberrypi:~ $) until the command is finished. In other words, the script never reaches the further lines in your script, but is stuck there.
To start the command, then move on, just add a
& to the chromium-browser command. Like this:
chromium-browser <url> --start-fullscreen &
Bash will start this command, then go to to the next one.