If you already have a similarly sized tablet, you could just buy a dummy HDMI plug, a few dollars, to add a second virtual desktop and then simply cast that screen to the mobile device.
There are pretty nice Android tablets now with 2.5k 120 hz HDR OLD screens. You can just connect it directly to the computer via USB, enable USB network tethering, then use something like the Moonlight client app with Sunshine screen casting server. With the wired connection, and a high bit rate such as 150 Mbps, you can get single digit millisecond latency and hardly tell the difference from an native HDMI display.
Tablets like those might be on the high end, but at least you'd have nice secondary display that's a bit more multifunctional. Or just go with a cheaper LCD based tablet or old iPad, if color accuracy, refresh rate, or resolution isn't a priority.
Looks like Moonlight does have their app up on the Apple store or iOS, and Sunlight has binaries for most operating systems. Personally, instead of Sunlight's server, I still use Nvidia's GeForce Experience software to stream games, as it takes less effort to configure. Of course, Nvidia may not be applicable if you're using integrated or AMD graphics instead.
Although, with Nvidia recently deprecating support for it's shield device, Sunlight provides support for the same protocol that Moonlight was originally developed against, but it's also open source. I've not used multi monitor streaming with GeForce Experience, something Sunlight would be much more flexible in configuring.
As for connectivity, I'm unsure if iOS supports the same USB network feature that Android has. I'd imagine at least the iPhone would, as that's a core feature/option for mobile hotspot connectivity, but maybe that's nixed from iPad iOS? Alternatively you could get yourself a USB C hub or dock with an ethernet adapter and pass through power delivery, so you can connect your iPad with a wired network and charge simultaneously.
Or you could just use Wi-Fi, but with wireless networks dropping and retrying packets, that'll impact latency or bitrate quality when casting displays. Although for something mostly static like discord windows, that's probably less of an issue. Windows 11, and maybe 10, also have a hotspot mode, where you could share your wired network via your PCs wireless radio via and ad hoc Wi-Fi SSID. That could reduce latency and improve signal reception, but you'd have to start the hotspot setting every session or whenever the device disconnects from windows' hotspot for more than 15 minutes or something.
You could try other remote display streaming software as well, like Parsec. However they have a online account login requirement with the freemium model, so I prefer the open source client Moonlight instead. However parsecs a lot easier too use when streaming from outside your home, or when remotely single screen co-oping with friends, without having to configure firewalls or domain names.