The main battery life killers seem to be the full-screen always-on display of many smartwatches and GPS. If you can dump those you can do a lot better it seems. Though in my case I have no interest in a fitness tracker without GPS.
I use a Mi Band which I wouldn't really recommend on the whole, but it gets great battery life and one reason might be that AIUI it piggybacks the phone's GPS rather than doing GPS tracking itself.
I'd love a Steel HR but 225 euro (~$250) for the one I want :(