@RobertBaker there has been some discussion around this topic here too.
One thing I and many people do to keep the phone GPS constantly updated in airplane mode is to keep a 3rd party app that directly queries the phone’s GPS, like Google Maps or GPS Status, running in the background. In airplane mode the additional battery drain seems hardly noticeable (except maybe with Google Maps).
If using the iNaturalist phone app, many folks also recommend permanently turning off the Auto-Sync setting in the app. This gives you control over when observations get uploaded, so you can wait for WiFi, or a consistently strong cell signal.