Using an iPad as a second screen

Not sure why it took me this long to learn how to do this (it took like 3 clicks).

The Displays control panel in system settings

Update: And then, inevitably, I started accessorizing. To put it at eye level when used with the arm-mounted monitor on my sit-stand workstation, or with a laptop elevated on a Moallia laptop stand / riser, I found this AiFeelife Height Adjustable Portable Monitor Stand to be the best compromise of quality, available height, reported durability, flexibility, and sturdiness ($38; make sure you get the one with the wingnut-looking “adjustment knobs,” the reviews of the ones that are Allen key adjusted are fairly negative). It should also fit my 16" portable 2K monitor, though I haven’t tried that yet.

Dining Table Workspace: Portable monitor stand holding 10th gen iPad