What is Your idea behind that use case to move it to nvme? System may boot a little bit faster but You will probably just get better results using both eMMC and nvme (with this one for apps, and data).
U-boot and spi stuff update allows You to boot directly from nvme, that is useful for boards without eMMC and even sd card. You got eMMC soldered so it’s there all the time to be used. You could simply create ext4 partition and dd system there, then just update fstab. This was so many time covered here that You could easily find that, like here: Moving file system to NVME procedure?
For armbian script - try armbian-install.sh or armbian-config - they sometimes work.
Also test Your nvme for speed, depending on configuration You will get from 400MB/s to 1.2GB/s, full speed can be achieved with special pcie2 overlay (that also is rather untested on armbian or stopped to work on recent images). If everything is ok then do stability tests, sadly it’s not easy and often You will find board with kernel panic after some time 