Tl;Dr use a second android phone to host movement and breathing detection.
This would allow use of sonar with headphones. Or allow a phone that isn’t under pillows and blankets to also do the alarm things.
So I’ve been thinking about sensor options. Sonar is no good (headphones). So I was back to putting the phone in bed with me, but it gets hot, AND I have sometimes been waking up from the backup alarm because it’s muffled.
But I, like most people these days, have an old phone, and it suddenly occurred that an option to delegate to a second phone for the accelerometer and/or sonar (and possibly alarm also, especially vibration) only, over WiFi or BT, might be a really good alternative. It could even be just the main app with a setting switch for ‘sensor device only’ that basically disables all the rest of the app and makes it discoverable to the master phone.
The biggest problem I see would be that it’s basically like a wearable in settings etc, but the word ‘wearable’ doesn’t apply.