you can programm custom apps on garmin with their own programming language monkey-c.
i think you can legally hardcode the http protocol at the backend to control the smartphone actions via webhook. everytime you add a function you need to sideload your program manually over the pc and then the program works only when executed, so you need to go into your created application to execute it. eg. program a webhook on one button.
the battery life and the solar panel were my usp for the watch. coros is close to them.
its enough for me to charge a phone only once a day, charging a smartwatch too is overkill for my purposes and my comfort.
the newer samsung watches using the google watch os, you might have luck with webhooks, but i think you need to save the webhooks as shortcuts somewhere on your watch and paste them in the browser. i am wondering if you find a cool solution for this, i am looking forward. you can send sms from your watch that can be read on your smartphone macrodroid as notification.
some smarthomes using ios support for the apple watch, i guess this will work with some android watches aswell.
as i wrote earlier i am using my watch only for activities, often in remote areas.