Making A Portable PC From A Minisforum UM790 Pro 

Don't doubt how much more compact I can make the solution if I used that, but it's a bit expensive for just a monitor and a keyboard.

@@marauderz thats true, another alternative is just to try to make lapdock from a broken laptop, but I guess it will take much more efforts than your setup (like getting hdmi-in pcb and connecting it to lvds connector of the display and making up the keyboard and touchpad into a basically a usb-c dock).