The Bangle.js 2 is pretty cool

Really wish we'd get some nice, fast RISC-V base boards with a nice amount of flash memory paired with a cool round LCD display..keep patrolling CNX for the parts but they don't seem to be quite there yet.

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.

I use the AmazFit Band 7, the last sensibly sized watch that exists it often feels like.

Weather fails to sync, but then it's probably the least important feature on a watch. The only feature I really wish Gadgetbridge could do that even the official stack can't is "nap mode"

As a narcoleptic person still recovering from major depression, I wish I could either press a button to silence the watch and set a "smart alarm" for 30 minutes. Even better if it would turn on automatically if it detects me sleeping during the day!

The only other thing GB can't do is stand in for the phone-side ZeppOS API functionality, but who needs that, let's be honest!

Fantastic battery life to boot. I have gone two weeks after forgetting to charge it while wearing it almost 24×7!

Idk if it can be called a smartwatch, but I just found out about the sensor watch, and now I really want one. Basically a hackable Casio f-91w

I just picked up a banglejs 2 and I love it. I was using a galaxy smart watch 5 but didn't work without gapps on my lineage phone. Its obviously not as good as the Samsung smart watch but I've been super happy with it. No creating accounts, getting tokens etc. Just pair it via Bluetooth and gadget bridge and you are good to go. Its a little pricey but for open source watch its awesome, I've heard good things about pinetime as well.

I only know of the Pine Time, however they warn that their watch OS is community driven and under active development.

PineTime is nice, wearing it right now.

Big fan of the PineTime for minimalism and extraordinary battery life, but the Bangle looks compelling. Maybe once the PineTime dies.

Now I miss my Pebble and Pebble 2 :(