I had severe issues with the UEFI BIOS being stuck in an upgrade process every time the Jetson Xavier NX would boot after I used the debian update instructions (so using sudo apt dist-upgrade to get 5.1.2 on my 5.1.1 NX).
I also ran into the APP partition not getting sized right by SDK manager while installing the 5.1.2 version of JetPack with it.
I did manage in the end to get it to install JetPack 5.1.2 with the use of the full capacity of my SSD through SDK Manager, by modifying one of the scripts. The entire saga (with instructions) can be found here:
Since this does wipe the contents of the SSD, I would advise you to backup any important data from your SSD before attempting my solution!