I read that Duet Display had been updated and that it was much better.
I tried it and it was still buggy as hell - using only a small amount of my iPad Pro screen and there were artefacts and it was still quite laggy.
So, I uninstalled it from my Mac and now my MacOS is broken and not functioning well at all. All the audio and video devices have disappeared and even my external monitor is not detected correctly. Luckily everything is backed up and I am in the process of nuking it and reinstalling macOS because I can’t find any fixes online.
If anyone is tempted to try it, at least wait until a new version is released that doesn’t break macOS!