It is not possible to zoom out on the map with your thumb while holding the phone with one hand.
You can zoom in with a finger or thumb by double tapping, but you can't zoom out with one finger or thumb.
Please add the this feature, which is already in almost all map apps. I know that Apple makes it very easy in iOS (I have done it and I’m just a lone developer). Double tap and slide up/down to zoom. Try it on your phone in Google or Apple Maps and you’ll instantly see how powerful it is.
The current zoom function is extremely cumbersome. You always need both hands to zoom out. At the very least, please add zoom buttons. Thank you!