@knebb, I have been successful with my Axon device couple months ago.
Here is my /etc/apt/sources.list.d/unipi.list:deb https://repo.unipi.technology/debian bullseye main
These are the steps I followed:sudo apt update sudo apt upgrade sudo apt full-upgrade
I didn't remove any packages after full-upgrade and simply rebooted with a backup in place. After restart, I removed all obsolete packages but those which had unipi or axon in name (see later).
Currently, I'm still using the kernel from Buster because the kernel packages in Bullseye appear to be broken. If I understand correctly, we should be able to use unipi-kernel-modules-dkms with the normal Debian kernel. Unfortunately, it currently has broken dependencies, namely the unavailable unipi-kernel-headers and unipi-os-configurator-data.
Here is the list of obsolete Buster packages I have left on hold:unipi-kernel-modules unipi-common axon-kernel-headers axon-kernel-image
Since I don't currently need a new kernel, I'm not in a rush to resolve this. However, it would be great to receive an official update.