Unfortunately, with sports watches, there are no standards like there are for Android/iOS…. Each watch manufacturer seems to do their own thing.
It’s also a very fragmented market with Garmin, Suunto, Fitbit, Samsung, Polar, Apple…
I’m assured that Garmin are the market leader in Sports Watches for athletes (and Apple being the watch leader overall).
The watch app is not all that big/complex, but the current version does rely significantly on Garmin Connect which may not translate for other brands.
I don't have any immediate plans for any other watch type at this stage. Given that each watch has its own development tool set, if we were to do another, I would certainly contract out that work. So to some extent the question comes down to funding.