That is a difficult question - and there’s no easy answer.
I see the dilemma when
- the old efi partition is on one disk
- the root on a second
- you want the whole #! on a third
Furthermore you have some some hard-to-recreate configuration in the old root - perhaps even data you want to migrate as painless as possible.
Assuming you have a working installation on the nvme device, the efi partition already contains the necessary efi loaders.
Assuming you are in posession of a bootable ISO with Manjaro. The ISO’s live-overlay usually contains clonezilla.
A viable option is booting the live system then use clonezilla (partition to partition) or dd to clone the old root partition from the usb enclosure to the root partition of the nvme. Something like
sudo dd if=/dev/$USB1 of=/dev/nvme0n1p2 status=progress bs=10M && sync
When this is done eject the USB and chroot into the system on the nvme and use efibootmgr to adjust entries as needed.
You may have to use e.g. gparted to bring the cloned paritions filesystem table in sync with the disks filesystem table.
Please be responsible - I may have missed something - in any case - you sitll have the original to fallback to