Personally, a 4k OLED with its own battery connecting either by USB-C or mini-HDMI

Brand name: INNOCN, they also have some with an integrated wifi so you can connect using the wireless display options of Windows (win-K for Miracast)

Wow, just googled it, looks fantastic.

It doesn't just look fantastic, it really is!

Well looking fantastic is the one thing a monitor should be good at :)

I still have a cheap Lenovo 1366x768 external. Not great but better than nothing and I can't afford 4K OLED goodness right now :'(

When I was doing this, I would just use a tablet.

Android, iPad, Windows, they all have monitor extending software and have for at least a decade now.

Saves you the need to by a screen if you've already got a tablet lying around somewhere.

Can you do this over a wired connection on Android?

Not the OP, but I have tried Duet Display on Android and it only works via Wifi. Samsung Galaxy Tab S7 (the premium price series) can be used as an external monitor, but only for Windows machines.

Thanks, I wonder how vulnerable to sniffing the wifi solutions are.

An Asus ZenScreen.