So, ended up purging all xtrx-related packages.As expected, kernel packages were able to be properly installed and configured.
GRUB configuration is also back to normal.Like Diederik said, this is actually the same issue as in bugs.debian.org/1038003, which makes this bug report a duplicate.
FWIW, I will be paying a lot more attention to my upgrades from now, especially when sleep-deprived ; sorry for the inconvenience.
