Depending on what you mean by "smart watch": you can try the Pine Time: https://www.pine64.org/pinetime/

51 USD will get you a sealed smart watch and a dev kit :)

Can I write my own apps for the sealed version?

The dev kit seems a bit too low level for me

https://github.com/daniel-thompson/wasp-os

i have personally made a metronome app for it

https://gist.github.com/mishushakov/7af0c459178f152a27005d31...