Tip would be not to charge when on the move as the vibration can cause the usb socket on your GPS to fail. Build into Cafe stops and your routine when you stop for the day.
If you can, an iPhone you can set an automation to send yourself a text message when the battery drops below 20%. That will appear as an alert on your Garmin Explore if enabled and enable you to charge phone before it runs out. You can also set an automaton to turn the phone to aeroplane overnight to further extend battery. Plus consider which apps you really want to have mobile data access to extend battery. Just 30 mins will get modern ones back up to 50% which should see you to your overnight stop. iPhone batteries are about 1500mAh, thus divide power bank capacity by that to get a rough idea of how many charges you can do.