iPhone Bluetooth volume is too loud even at minimum. Same headset volume can be much lower on Android.

I have found a solution (or rather a workaround) for audiobooks etc. 

Settings->Accessibility->Audio/Visual section:

Select Mono Audio and reduce the volume of one earbud with the help of BALANCE. Simply forget about the opposite earbud, do not use it. Problem solved! A kind of...

Unfortunately, I found this method a bit too late, after purchasing a cheap Chinese UMIDIGI G3 Plus - exactly because I was not able to reduce my iPhone volume to fall asleep comfortably while listening to my audiobooks. Although no regrets at all because this phone is cheaper than Apple AirPods and has so many features not available on iOS(even besides a 1TB SD card slot). For example, I can use a special equalizer app which changes the sound of YouTube or any other app not equipped with its own equalizer. No such special apps for iOS, period. On my new non-Apple phone, Bluetooth headset volume can be changed separately using volume buttons on the headset and on the phone (configured in Developer Mode). On iOS, you click Bluetooth headset volume and see how the phone volume changes - less precision.

Apple programmers could easily fix this volume issue - but they obviously prefer to force people to migrate from iOS. 

By the way, the best volume control for those who prefer minimal loudness is not buttons but a wheel. Just buy a 3.5mm cable with wheel volume control for $3 and insert it into your normal phone, or in the case of iPhone buy a cheap bluetooth dongle and insert this wheel cable into its 3.5mm hole. Yes, sound quality will be rather mediocre or even mono with a cheap bluetooth dongle - but you will be able to set the desired audiobook volume VERY precisely and effortlessly, as turning the wheel is much easier than any other method.

Anyway, I hope that Apple will let me know when they will fix this loud volume issue or even better decouple volume on the iPhone from headset volume. Thanks a lot in advance for your concern about customer satisfaction!