I didn't like the look of it. Too strangely compressed. But good point about space and resource saving!
The school only had Arduinos, and this is my first contact with other microcontrollers than RPi. So it was really no choice.
Can you recommend a suitable one David?
I've so far made so much progress that it successfully runs on my Nano, which is great 
Thank you all for all your help guys!
A little off topic perhaps, but I really want to change PWM frequency also for the fan. It makes a bunch of noice on the default Hz..
From my research (link) it seems the better option is to setup Timer 1 to produce 20kHz PWM on pin 13. But I also read it "can have unintended consequences" so I'm a little bit scared
Maybe someone can help me figure out how to best write this? I'd rather not destroy my microcontroller by just trying myself if I can avoid it