Earlier quoted context omitted.
I feel like I'm starting to sound like a fanboy - but it's because I've had other smart watches and it's just so much better . Why not consider garmin? Not sure if their SDK is going to hit the mark as I'm guessing you want complete control and it's definitely not that, but they have everything else you've asked for. https://developer.garmin.com/connect-iq/sdk/
Well, they're three times the price. That's a bit hard to swallow, honestly. A Pebble is a nice gadget. The Garmin (looking at garmin.com, clicking the first thing that looks like a watch) is close to a month of rent. I believe you when you state that you're satisfied. On the other hand, I really don't feel that this is in the Pebble replacement league.
Been badly burned by Garmin in the past, but maybe I'll take a look. They seem to be competing with Suunto, but without that ridiculous iOS only thing.