Okay I managed to make it work, there was another typo in step 7, this is the correct script to make it work:
#!/bin/bash
# Place custom startup commands here
sudo -u www-data /usr/bin/php /app/data/mixpostapp/artisan horizon &
sudo -u www-data /usr/bin/php /app/data/mixpostapp/artisan queue:work --queue=high,standard,low --sleep=3 --tries=3 &