I just checked out Amazfit and they seem to have products covering much of the price spectrum. I too, use my watch to sleep, though my newer one[well, technically, the model is couple of years old but Galaxy Watches are expensive at launch :p ] always shows that I get much lesser deep sleep than what I got with my previous basic fitness tracker.
About that, when I switched to Gadgetbridge app, I got more deep sleep reported compared to the native proprietary app caled ZEEP. I can't really know which one is more accurate, But I always felt that the ZEEP app figures were too low.