Rather than all-or-nothing (full features or Power Reserve) I think there's a decent middle ground. By disabling Hey Siri and Heartrate Monitoring today I ended the day with 61% battery left which is double my usual level. I prefer pressing the Digital Crown to using Hey Siri and Heart Rate monitoring is most useful when I am doing cardio and I didn't do the gym today. So I disabled them and I am amazed how huge their impact is. So I suggest if you're burning through the battery one day and/or have forgotten your charger disable these two and you can probably squeeze 2 days out of your Watch with minimal adverse impact (especially compared to Power Reserve mode).