There appears to be three apps available:
Any watch compatible with any of those apps, should work.
The best watch that isn’t a pre order is probably the Bangle.js. It can be bought in the US for $150 from adafruit. If you don’t mind waiting, the Pebble is probably nice too. That’s assuming it works with RockWork. Not sure the new Pebble watches work with it, so you may need to write your own app.
If I get a watch, I’ll probably get a $40 preowned watch I can put AsteroidOS on. Probably won’t have a heart rate sensor, or maybe even step counting, but I don’t care. Or maybe you just can’t sync the data to your phone. Don’t want to spend $150. Says it’s been used two times I think. Might not be available when I get more money though, if I do.
Another option would be my Amazfit, if it’s the right one. But I don’t know where the charger is. Probably needs a new battery since it’s been sitting dead for years. Easier to buy the $40 watch. No idea how much a charger and battery for my Amazfit is. Also, don’t know how hard it is to open.
The $40 watch, says you can open it with a screwdriver I think, so replacing the battery on that is probably easy.
I’ll probably leave the Amazfit where it is, and never look at it. Maybe donate or recycle it. Or keep it just in case.