The circuit board will break down into these main parts:

-24V to 5V Power for Nano and other ICs

I am using a buck converter module for this.  The output is settable with an onboard pot.  The switch will be placed inline before the input to this module.

-24V Switch circuit to supply power to the iron

I will be using a IRF540N MOSFET to handle the current/voltage.  As a buffer between the Nano and the FET, I decided on a MCP14E7 MOSFET driver.  This is probably overkill, but I had some on hand.  We aren't switching this FET quickly and the FET is only handling 2A.  You could probably use a transistor or another FET to interface to the Nano, but I wouldn't control the FET directly from the Nano.  #GreatScott! has a great video on driving MOSFETS .

-Voltage divider and amplifier circuit to bring the thermocouple resistance feedback into the Nano's ADC

For this I am using an LM358 OpAmp.  The iron's resistance is around 1 Ohm at room temperature and about 20 Ohms at 400C.  Because this resistance is so small, I need an amplifier circuit to get better resolution coming into the Nano's ADC.  The LM358 has two amps in one chip, so I will probably use a voltage divider and the other amp to monitor incoming power voltage.

-Potentiometer feedback for setpoint

Most potentiometers will work here to create a voltage divider for the Nano's analog input to create a way to control a setpoint temperature.  I would use something larger than 2K Ohm.  I am reusing the 500K Ohm pot that is from the old soldering station.

-OLED display connection and I2C communication

Simple 4 wire connection to the display, SDA and SCL from the Nano, power and ground.  I'm also including pullup resistors for the SDA and SCL lines, but I need to meter the OLED and see if they are already present on the display board.