Not what you're asking but on the opposite side you might want to check out gadgetbridge.

Gadgetbridge is a free and open source Android application that allows you to pair and manage various gadgets such as smart watches, bands, headphones, and more without the need for the vendor application. So in short, you can use Gadgetbridge instead of relying on your gadget's own proprietary app.

Maybe a Colmi smart ring, versions ranging from 13 to 30 €?

I got a Colmi Smartwatch for 13 €. It doesn't integrate into the standard Android Health thing but seemingly uses its own protocol or sth.

Didn't pebble release a smart ring? I don't know the price, but from what I understand their newer stuff (since the relaunch) is either fully or mostly open source.

I don't know if it includes the sensors you want though.