I was about to write the same comment but you expressed it much better. I would also be happy if it could be both an android tablet but have a monitor mode where it just becomes a monitor.
Plus one on this. Would be amazing if there was a display mode possible or just a variant with a usb to edp board inside without the android tablet bits / battery (even if it meant losing the wacom digitizer in the process)
You can use VNC to make an android tablet into a 2nd monitor. The latency isnt good enough for playing games or anything but for web pages and documents it works great.
I've recently been using a piece of software called RustDesk, which so far feels pretty performant, better than VNC, RDP, TeamViewer, RealVNC etc.: https://rustdesk.com/
You can host a relay inside of a Docker container if you need it as well and it even supports AV1. There's also password auth for each device and permission control.
They have a mobile app too, though I haven't used it as much as the desktop variety, might be worth a shot.
