I work with Rayneo Air 4 Pro - they have HDR/120fps displays (image quality is top-notch and they're bright enought to work even with direct sunlight). They don't have battery or software, it's plug-in-play USB-C external screen essentially. It's more than enough for travel, or for sitting straight while working cafe, for privacy reasons or working outdoors. I even tried them while walking in the forest (with laptop in the backpack and using voice input).
Drawback - they don't do 3DoF tracking, so the screen just follows where you're looking, you can't change screen size, and no multimonitor setup. They have IMU sensors, though. It's possible to create your own virutal monitor software (or use someone's else like RealWarp), but IMU drift is hard to beat and experience is far from perfect. So I stick with 'just one high-quality screen' for now, and to be honest it was some quality-of-life improvement for me.
PS. Couple of weeks ago there was new release from RayNeo, which supposedly have 3DoF and same quality. Haven't tried those.