I have a portable monitor that I'm pretty pleased with.
It has a magnetic cover that goes over the screen to keep it safe, and that same cover folds and goes on the back to act as a stand when it's in use. Power and video are via the same USB-C cable.
Nice and slim and stays in my bag most of the time but when I want a second screen I can whip it out in two secs.
A screen that attaches to the laptop sounds convenient initially, but I feel like in practice it would be a hindrance and make your laptop clunky and bulky.