Funny, I was just researching this last week. I wanted to use my Galaxy Tab S6 as a secondary display. There was some recommendations for Duet, but I guess some recent updates seemed to have really messed up this app or something. So I didn't try it.
I ended up going with spacedesk.
Multi Monitor Display (USB, WiFi and LAN). Screen Sharing (Mirror and Extension)
play.google.com
For basic wireless connectivity it works amazing. I had to install an app on my host PC. And then I add my PC to the Android app and connect. Windows treats the Android tablet as a second monitor. I can change resolution and scaling, change the position of the screen, all the usual stuff you can do with a regular attached secondary monitor.
I only ran into two problems.
First this says it supports USB tethering, but I could never get that to work. And their support forum seemed to indicate you'll still need a wireless connection even if using USB tethering. Seems a little complicated and maybe not fully functional.
Second is I tried this on a work issued laptop with some firewall restrictions and I could never connect to that laptop. I didn't troubleshoot this a lot though, perhaps I can use a different port. But I didn't really need it on that laptop so I didn't pursue that further.
Overall super impressed that this is a free piece of software and works really smoothly. I'm using wifi and have all the graphics settings maxed out and the display looks good and still pretty fast for web browsing and apps (I haven't tried gaming or watching videos on the secondary display).
Facebook
Twitter