I might have to retract the “flawless” part of my “sales pitch”. I tried to make it spontaneously work on my work laptop using Fedora and it did in fact, not work. To visually show how it should work, I made a picture with an iPad where it actually does work with my work desktop with 2560x1440 resolution, 60fps. But, it did worked in the past, so I am fairly certain I just suck at using my device. If it would work it appears just as any other capture card…as weird webcam device.

But as said, you need a splitter before the card and then one output into the card. I actually spend a minute to locate a non-black HDMI cable just for you! https://imgur.com/a/BvZDZRJ

There is the avermedia bu113 card, around 100 euros, works on everything from Windows pc, to ipad and of course linux without any additional drivers as it supports with UVC, downside, you basically need a hdmi splitter if you want to see something as its build to use big cameras as webcam

This one from Hotline Miami https://www.youtube.com/watch?v=SNE2oCZH_4k

Interesting, but what does this solve what Bookstack does not solve? I mean sure, it looks nice and hacky and all that. But if i am going to host some note thingy, https://www.bookstackapp.com/ is right there and apparently the dev nowadays lives from the thing (which is nice i guess). Not to belittle your project in anyway, even if something like your thing would exist exactly as that its still commendable but i am already running Bookstack and this seems to add anything to any use case i could think off.