As I’m having MacBook issues with my overworked 2017 I’ve had to switch to my Apple Studio / Mac mini rack mount rig and needed a portable monitor solution. What I’ve gone for is a couple of ViewSonic VA1655 15’6" screens and added backwards facing USB-C and HDMI adapters. I will make a custom flatpack desk stand for the screens to help with height and tilt adjustment as well as cable management. Note: there are cheaper options which will work just as well as these are basic IPS screens.
Carrying an Apple Studio or Mac mini in your backpack is doable but when you add a regular sized flat display this is when it becomes a pain. Portable screens like the ViewSonic are the solution.
Upside: small and portable, USB-C and HDMI video input. Downside: standard pixel density, 1920x1080 resolution (reduced desktop real estate).