These display modules can either be SPI or I2C. It sounds like yours defaults to I2C. I2C connects to the SCK & SDA pins. They should normally have the required I@C pull ups on the display board. DC in I2C is used as an I2C address selector. If it is pulled low then the address should normally be 0x...

I think that even for the 2n2222A version it is hard to come up with a worst case design that provides sufficient Ib to saturate and yet doesn't potentially try to draw more than 40mA from a GPIO. 2n2222A max Vbe Sat @ 500mA Ic 2V might be a bit less at 300mA but not actually specified Assume GPIO V...

Just to amplify further why I think trying to exploit potential 40mA capability of a GPIO would be a bad idea. 1 Vbe sat of a 2n2222A could be 2.0V 2 Vo of a GPIO at 40mA could drop to 2.6V 3 So to guarantee 30mA Ib what base resistor would you use? 4. Then under typical conditions where GPIO might ...

When using bipolar transistors for switching you do not use the current gain (beta) to determine appropriate base current. That parameter is specified when the transistor is in its linear region with a significant Vce. Gain drops off dramatically as Vce falls. The normal 'rule' for switching is that...

Using a bipolar transistor like a 2n2222 is not a great idea for switching the current to the solenoid. For a 300mA solenoid you would need to feed about 30mA into the base of the transistor to saturate it (switch it hard on). That is to much for a GPIO pin. You could use a darlington transistor lik...

As has been pointed out a series gate resistor does very little when using a MOSFET driver. The only possible benefit would be to add a degree of protection to the GPIO pin if there was some catastrophic failure in the MOSFET device. These tend to be very reliable devices and I do not add a series r...

You can use the TXD and RXD pins as GPIO.

Obviously they are not operational during a flash upload operation which may mean swapping wires to do uploads but if you use OTA then that doesn't matter after the first flash.

Using RXD doesn't affect any serial debug print but using TXD will.

If you use an esp32 module with 4MByte PSRAM (e.g. WROVER) then you can store your whole capture in a buffer before writing out to file.

I don't know why GPIO14 would affect the boot unless it is something to do with the camera software. Maybe try it with a minimal sketch to see if that boots OK with GPIO14 pulled up. On my camera app I do use the SD card so there is little IO free. I use RXD as GPIO3 pin; just use pinMode in setup.....