Thank you for the answer.
The problem was I accidentally removed the
/boot folder, WHILE trying to back it up. So there were only
OS folders and something was missing.
I figured out later that kernels are also stored in /boot and tried to reinstall the kernel manually, but ran into a number of minor problems.
Though I also found out that Timeshift keeps not only /root but also /boot folder (even if it is physically on the other disk)! Thanks Universe
So I just copied files from the timeshift /boot backup to the new ESP folder and GRUB showed the OS correctly.
There were also some minor problems which I solved with apt update/upgrade while being in chroot.
So, the problem is solved. At least it looks like that for now, as everything works fine i guess.
But i must mention that
this passage from Debian Wiki seems incorrect
Bind mount various virtual filesystems:
# for i in /dev /dev/pts /proc /sys /sys/firmware/efi/efivars /run; do mount -B $i /mnt/$i; done
And copy /etc/resolv.conf to mounted disk, to make the internet work under chroot.
I will try to suggest an update in the Debian Wiki, if it is possible for a newcomer there.