I had a similar problem and had deactivated the backports. I then installed the 6.1.0-33-amd64 kernel - not the Proxmox kernel.
However, after some searching I found that my ‘sources.list’ was not complete and therefore the appropriate packages could not be found because the corresponding requirement for ZFS was not configured.
ALT
deb http://deb.debian.org/debian/ bookworm main
deb-src http://deb.debian.org/debian/ bookworm main
# bookworm-updates, to get updates before a point release is made;
# see https://www.debian.org/doc/manuals/debian-reference/ch02.en.html#_updates_and_backports
deb http://deb.debian.org/debian/ bookworm-updates main contrib non-free
deb-src http://deb.debian.org/debian/ bookworm-updates main contrib non-free
NEW
deb http://deb.debian.org/debian/ bookworm main contrib non-free
deb-src http://deb.debian.org/debian/ bookworm main contrib non-free
# bookworm-updates, to get updates before a point release is made;
# see https://www.debian.org/doc/manuals/debian-reference/ch02.en.html#_updates_and_backports
deb http://deb.debian.org/debian/ bookworm-updates main contrib non-free
deb-src http://deb.debian.org/debian/ bookworm-updates main contrib non-free
I was then able to install the missing package ‘zfs-dkms’ so that the ZFS modules could be built to match my kernel. The kernel sources must of course also be installed beforehand.
The ZFS service ‘zfs-zed’, which previously could not start due to a lack of kernel modules, then also worked.
However, this means that different licence types are used and this is displayed accordingly during the installation.
I can now test again whether this also works with the backport kernel.