Wiring of my laser

Are you married to using Marlin on the controller? If not, I’d suggest that you consider a GRBL based solution.
JTech recommends using D9 (extra fan) for laser PWM control.
The problem with most Marlin solutions that use either the fan or bed heater pins is that PWM frequency is typically quite low.
The Arduino Mega should be able to be configured for a reasonable PWM. You want 1000Hz minimum for a diode laser. You should be able to configure Marlin at compile time to enable faster PWM but may not come that way by default.
Note that while M106 will work ideally you’d want to be using one of the more advanced control schemes like Inline.