@Warp And this time, use btrfs and separate home.
Make sure timeshift is setup and that an update-grub finds them.
That way you would never end up in this situation again. Then you could have used a live iso, chroot, install grub and let grub find the timeshift subvolumes to boot/restore from. Also install kernels if needed.
But a timeshift is not a complete backup!
I read the whole thread now and it seems you are accepting your fate and take it as a learning experience, well done!