Well, I took my chance installing directly the “Alternative Ubuntu for Joule” on a second Joule not flashed (since this way I did have acces to BIOS to boot on USB), and it works properly… (ROS install and turtlebot teleop working with no problem)
Seems weird 'cause there must be a reason the guide tells us to flash before doing it, but i am not complaining, it works.
I’ll keep trying make the first one work, and come back to you if I find a solution.
Thanks @WillSon