I would recommend cloning the entire disk to another disk of equal or greater size before, best procedure is to boot to a USB installation, run Gnome-Disk-Utility, create a disk image onto a second larger disk, then restore that image to a third disk which is equal or greater in terms of capacity to your bookworm disk, then unplug your orignal bookwork disk amd then attempt to boot from that third disk (fingers crossed)

If you’re comfortable with the dd command that’s another route to take but if you’re not paying attention you can very easily wipe your own disk!

Is it LVM2? If you need a GUI I recommend: Blivet-GUI from a bootable USB environment so as you said nothing is mounted, hope this helps!