Would it be clever and wise to learn the codes/command lines of PureOS is based on, to start with knowledge in backpack once the phone gets in the hands, so you can start right away doing stuff?
It couldn’t hurt, but I don’t think you’ll really need the terminal for day to day phone usage.
You can install a Debian-based distro (Debian, Mint, Ubuntu, Pop_OS!, PureOS) on your PC to start learning the commands.
However, when I install the Librem 5 image in a virtual machine on my PC, it doesn’t look you will need to use the command line very often. Linux/Wayland/GTK/Phosh + GNOME mobile apps look to be finger friendly, just like Android, iOS and Ubuntu Touch.
Typing commands on a virtual keyboard in a phone screen is slow and tedious. When I want to use the command line in Android, I use an external Bluetooth keyboard, and I suspect that you will want to do the same with the Librem 5.
True, it’s the same about typing messages in a online PlayStation game which is frustrating, that’s why I use a USB keyboard here.
Or SSH in.