In diagnosing the problem he realized that his circuit connected to GPIO_0 was inadvertently putting the ESP8266 into programming mode, since it was holding the pin LOW unless the intercom button ...

I/O is limited. You’ll have to work with that. The C3 is really a blend of the two: a limited number of GPIO pins like the ESP8266, but with nice peripherals like the ESP32. If it’s priced to ...