When building one-off projects on perf board, strip board, whatever your method, do not build the entire unit, apply power and hope for the best. That’s a recipe for failure because when you’re starting out, you will make mistakes. Mis-wire a power supply and you will absolutely let the smoke out of something.
Build your circuit in blocks. Wire up the 24 vdc buck converter. Solder a resistor on the output. Power it up. Set the correct output voltage. Check the ripple voltage. Turn it off. Wire the Nano and the LED. Change the Blink demo to use the external led. Power it up, download the program and test. Get the Nano running. Install and connect the lcd. Write or find a hello world sketch and test the lcd. Build the opamp. Connect a pot in place of the thermistor. Download the final program. Test the opamp and the analog input. Only when the circuit works, add the mosfet. Then you can connect your soldering iron.
Se how that works? One step at a time. Get each functional block working, one at a time. Individual components you can test before installation. Resistors, caps, transistors and similar parts - test them before installing. Only when you’ve got some experience can you just throw the whole thing together and have a reasonable expectation it will work on the first power up.