Failed to disable unit: Unit file qubes-sysinit.service does not exist.
dpkg: error processing package qubes-core-agent (–remove):
installed qubes-core-agent package post-removal script subprocess returned error exit status 1
dpkg: too many errors, stopping
Error during installation qubes-core-agent:installed qubes-core-agent package post-removal script subprocess returned error exit status 1
Errors were encountered while processing:
qubes-core-agent
Processing was halted because there were too many errors.
sh: 1: /usr/lib/qubes/upgrades-status-notify: not found
E:Sub-process /usr/bin/dpkg returned an error code (1)
There is no Qubes OS debian repository for unstable (sid), but there is one for testing (trixie). And trixie-unstable is just incorrect distribution name.
The packages in testing are newer than the ones from stable (bookworm) repository so the chance of the package conflicts are lower.