As it turns out, if you have one of the new Mac minis, you can use an iPad Pro as a display with Astropad's Luna Display dongle, which is designed to turn an iPad into a second display for a Mac.
Luna Display hasn't previously been used as a primary display because it's been paired with Macs that come equipped with a display, but since the Mac mini has no display, it's the perfect machine for testing an iPad Pro as a primary display.
In a blog post, Astropad explains how the Luna Display dongle, the iPad app, and Mac app can be used together to turn the iPad Pro into a Mac mini display, with full instructions available.
According to the Astropad team, using the iPad Pro this way was like a "whole new product" because it's essentially macOS on the iPad with the Luna app, and a regular iPad Pro when not in the Luna app
This setup truly combines the best of both Mac and iPad, with the processing power of the Mac Mini and the edge-to-edge retina display of the iPad. Using Luna, we're able to take full advantage of every pixel on the iPad at full retina resolution. It offers more ways to interact with your macOS too, where you can seamlessly flow from mouse, to keyboard, to Apple Pencil, to touch interactions. And since Luna runs over WiFi, you have the flexibility of a completely wireless workspace. It all just works.To get the same setup, you'll need a Luna Display dongle to plug into the Mac mini's USB-C port, the accompanying apps, an iPad Pro (any will work, but best results will be seen with Apple's newest model), a keyboard, a mouse, and a reliable WiFi network.
The Luna Display can be purchased from the Luna Display website for $80.