My research also shows that the wrist is a less reliable place to take the reading than the fingertip. Also, it is necessary to keep still whilst taking the wrist reading. Further, the band does not have an infinite adjustment, rather a stepped one dictated by the holes in the strap. Therefore it is not possible for somebody to properly set the sensor for measurement without being painstakingly fastidious. These factors for me having worked it out in hindsight make the device less than sensible. And you are right, reading are best taken occasionally and deliberately.
Garmin's warning about battery life is very non-specific and not really well presented. Something that can suck half of battery lifenis quite significant imo.
I have just taken my reading using the finger device: SpO2 99, pulse 46.