This guide may be helpful in installing Zorin in BRTFS. Zorin 16 is built from Ubuntu 20.04.
To access the external harddrive from terminal, usually, you need only CD into it; the path would follow from /media.
If it must be mounted first:
When you say you need a simple window manager, I am not sure what your needs are. Zorin OS Lite comes with XFCE4 which uses a straightforward Window Manager (XFWM4). There is also Openbox, AwesomeWM and so on... All of which are Quite Simple. You may find that installing XFCE4 on Zorin 16 (current build) may serve your needs, or you can install some of the other simple and lightweight Window Managers available on Zorin OS 16 just as easily.
You can install XFCE with
sudo apt install xfce4
This may be necessary for some that want numlock on at login screen (It comes installed with Zorin OS Lite, but Zorin 16 Lite will not be released until much later this year):
I have no relevant experience in Video Editing to make any recommendations that are worth anything. Perhaps others can chime in here.