Web yes that is correct, your d1 mini and all esp8266 devices have a single analog input! Source: The wemos d1 mini has a 3.3v and 5v output to power.
Web the d1 mini has a micro usb for auto programming. If your project is ready for action you could put the wemos d1 mini on this custom pcb and more than one 3.3v pin and one ground for the pcb is not necessary. Arduinoer esp8266OLED, esp8266Arduino library for I2COLED displays from Not all gpios are exposed in all esp8266 development boards, some gpios are not recommended to use, and others have very specific functions.
