Check out testing:

  sudo apt update && sudo apt upgrade
  sed -i 's/buster/testing/g' /etc/apt/sources.list
  sudo apt dist-upgrade

Getting security updates from unstable (not required):

1. Pin unstable lower than testing.

2. Configure apt/debsecan as outlined here: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=725934