I am also open to modifying the ESP32 so that I can read the battery voltage. (Like connecting a resistor from the (+) of the battery to a GPIO pin of the ESP32)
Looks like, without actually having one in hand to examine, that the Vbatt+ would have tapped and put through a resistor divider and then the voltage read by an A:D and so on and so forth. I'd use one of the GPIO pins on the B port of the ESP32. All pins on the A port of the ESP32 can be configured as input/output. GPIO0 to GPIO32 are on port A, Port B are GPIO 33+ and are only configurable as input only, without pull ups/downs. Thus the GPIO pins 33+ are best used for A:D.