of course it's your money and you can do whatever, but, I personally would not spend money on a 2nd AW just for sleep tracking. Having said that, I actually have 3 AWs, I wear my Ultra 2 for ~ 38hrs continuously (incl sleep) and then put it on the charger and wear my S7 Ti for sleep tracking ... I upgraded and did not want to ell/trade-in my older AWs due to poor trade-in value.
It sounds like the SS is too heavy for you during sleep, I can appreciate that.
Rather than getting an AW you might want to look at something like an Oura ring for sleep tracking, there have been posts on here about those though I have no personal experience nor do I endorse them, but afaik they are <$100.
If you really want an AW, a SE should be sufficient for just sleep tracking and I'd go for just the best deal