PineTime is nice and cheap and does a little BlueTooth. It's easy to flash your own firmware to it; I think it might have been an option.

That said, Pine64 does not do a whole lot of work on their platform and I guess a lot of people are wanting _more_. It's a simple watch and for what it does and costs it's great. If you think you'll get an open source Apple Watch, you're going to be disappointed.