MorganS:
N-channel switching is usually used for 'dumb' loads like lamps and motors. The N-channel devices are cheaper/smaller/lighter for the same power handling. (Electrons more mobile than holes in the semiconductor.)It's not usually used for 'smart' loads like the Arduino. The reason is that the Arduino is connected to so many other things, many of which can provide alternative paths to ground. The Arduino won't be fully switched off if it still has a few buttons or switches connected to ground.
If you design the circuit properly, it can be done but for a one-off where you don't care if the transistor costs $1 more, it's easier to design with the switch on the positive lead from the battery.
That makes sense. Let's avoid wasting power unnecessarily, the pull down resistors in my previous drawing could be an alternative path to the ground.
As you said the components are really cheap (and delivery fees will be by far more expensive than the components), so I will go for a NOR gate + logic level P-channel FET.
Thank you very much everybody for your detailed contributions ! Can't wait to receive the new parts 