I have a fresh Jetpack 4.6.1 install which I modified to run from a USB (SSD) drive. Then I tried to follow your instructions and step by step and at some point I got some errors, it still went though but at the end, I have Ubuntu 18.04 LTS and the same version of Python.
After sudo apt-get upgrade, I get nvidia-14t-bootloader encountered errors while processing: Sub-process /usr/bin/dpkg returned an error code (1). Do you have any ideas on how to fix this error?