Upgrade Hassbian installation to Debian Bullseye in order to get python 3.9
Update OS. Follow another guide, except also remember to update all of the other /etc/apt/sources.list.d/ files too. :) Note that there is no hassbian-scripts repo for bullseye, so just leave that one at buster. https://www.tomshardware.com/how-to/upgrade-raspberry-pi-os-to-bullseye-from-buster
Update hassbian-scripts, which likely got removed at the automremove step - removing your systemd unit and all sorts of other things. The scripts package available on gitlab is broken, but there's a fork with a few fixes (primarily python package deps).
git clone https://github.com/dannysauer/hassbian-scripts.git
cd hassbian-scripts