Ah, I had misunderstood your /boot situation previously. There’s an easy way to fix it by backing up current content of boot, unmounting it, creating some dir somewhere where there’s space (/tempboot was my choice last time), bind mounting it to /boot and going through the apt process. Then unmount the bind, mount the real boot, delete everything except currently booted kernel stuff, copy all the things from /tempboot update the initrd and grub. Et voila!