Reuse old iPad as a Monitor

I bought a mini-computer as my second home physical computer and have been playing around with it, but I still want two screens (lol). When I was mainly using a notebook computer, a dynabook N40, I naturally had two screens: a notebook monitor plus an external monitor (a TV, though). The mini PC is a desktop, even though it is a mini, so of course it does not come with a monitor. The MINISFORUM UM350 seems to support Display Port and Type-C output in addition to HDMI output, so it can output up to three screens simply by connecting it.
My current desk at home is not very spacious, so the Toshiba Regza I use as a monitor is also positioned so that it is somehow outside the desk with a gas pressure arm , so I was looking for something portable and space-saving to buy another monitor, but then I remembered I have an old iPad at home. The iPad I have at home is what is called a fourth generation , which was released in 2012. It is not so old from the point of view of a Linux user like me (:-o), but since iOS only supports up to 10.3, most of the apps no longer work on it (;~s). I had to use a technic to install once on a relatively newer model with my Apple ID and then put the older version of the app on my iPad. But even so, most of the apps stopped working except Safari.
I did not expect much from such a situation, but it turns out that spacedesk, a well-known monitoring app, runs on iOS 10.3 as well! So the 4th generation iPad has started its second life as a monitor (lol).