There’s no reason to run nixos-generate-config if you already have a config.
Just boot live image, mount all your partitions relative to /mnt and then nixos-install again.
As a faster alternative to nixos-install: after mounting everything relative to /mnt, you can nixos-enter then run
NIXOS_INSTALL_BOOTLOADER=1 /run/current-system/bin/switch-to-configuration boot
which should work.