You can get similar HW for $10-$15, and not the easiest hack but looks doable: https://www.youtube.com/watch?v=w90RVspTkt8
I'm surprised Oura hasn't pursued using the ring as an input method as the guy suggests in the video. It'd be super cool to like double-tap the ring then use it as a scroll wheel when running and listening to music/podcasts, interact with AI models, etc.
I have a miband I use with gadgetbridge. I'm reasonably happy with the app, and it has visibly improved over the last year (it also wins by default being opensource + the only option for keeping data private) but the watch is a bit bulky when sleeping or typing so I stopped wearing it.
I can't imagine $10 hardware will be particularly accurate, but cheap price + data control is enough to give me an excuse to play with one.
[1] specifically rings intended to be used by the QRing app - https://gadgetbridge.org/gadgets/wearables/colmi/