My heart sank last night when I was cleaning and my almost new Gear S3 hit the corner of my Samsung TV (Ironic) frame corner. This caused a scratch to the watch screen and frame edge. It wasn't even a big hit, but I guess just at the wrong angle. So mad and frustrated.
That looks like a really deep scratch, there is no fixing that. Next time look for a device with a sapphire crystal, they are virtually indestructible. I have had my Huawei watch for over two years and it has taken all sorts of punishment. The screen still looks brand new. Even if you were to fix your current watch it would just get scratched again in relatively short order.
I guess the one thing you could do is get a tempered glass screen protector for it. It would cover up the current scratch and it might mask it somewhat. Buy a few of those actually. Then as they get ruined just keep replacing them.
You've got to get used to a smart watch taking some punishment, by the way. It's not like a phone that you can coddle with a case. My Huawei watch has a really tough grade of stainless steel but it still has lots of superficial scratches. There is no way around it.
