I remember someone mentioning this in a thread a while back
https://www.espruino.com/Bangle.js2
Your mileage may vary, based on what smartwatch, what model of pixel, what you want it to do, and what permissions you are willing to grant to let the watch talk to the phone.
As a rule of thumb, Bluetooth is kind of a no-go as far as security is concerned, from what I can recall. Its just inherently flawed in some fundamental ways that I don't understand, but Bluetooth is common, a wireless transmission, and not quickly updated between device iterations, and almost never updated within devices (headphones never get updated Bluethooth stuff), and thus is plagued by the curse of backwards compatibility, much like Windows is.
Personally, I just use a steel Casio g shock dumb watch with the ability to receive those sync signals from the various atomic clocks dotted here and there... and if I get a text, I just look at my phone or ignore it until I can.