Gatgetbridge (your link) has a breakdown of devices they support https://gadgetbridge.org/gadgets/ . You can click through the vendors to find devices which are both “highly supported” and “no vendor-pair”. Meaning most/all the features work without any reliance on the vendor app.
As for the similarity you are asking about with pixel->GrapheneOS, there are very few watches that can run an alternative open source firmware or operating systems apart from the ones that are already open source, like bangle.js, pinetime, etc. Wearables are even more specialized than phones, they require specialized code designed specifically for them and would likely require pretty extreme effort to reverse-engineer.
I use a pebble 2 HR with gadgetbridge but the watch it self runs the old pebble firmware which gadgetbridge talks to. This is fine for me, but if you are looking for a more modern watch you may have to make some compromises.
A relative of mine attempted to move from an iphone to an android phone, and generally move to other services on non-apple patforms. Not really self hosted, but I tried to help a bit in that area too, placing them on some of the things I run. The reason was their own, they saw pictures taken by a friend who had a flagship android phone and thought the pictures were amazing in comparison and wanted to try it. They were not even that deeply into the ecosystem, didn’t have watches or anything, just the cloud services.
Sadly what happened was that apple makes you feel so cozy and content in their UI design that anything even slightly different is really disruptive. It didn’t go well, and the experiment lasted about 6 months and then they were back on an iphone again. I would hear constant complaints and cursing about various UI differences. Touching parts of the screen triggered actions they weren’t expecting, and they couldn’t manage to learn the new workflows. It was sad to watch, and I hope things go better for you. In my opinion Apple is really good at attracting people and slowly convincing them that they don’t have to think for themselves anymore, so then when they try, they no longer feel competent enough to do so.