I've run wires through styrofoam walls using the common household wire puller to penetrate, pull a string, rope and then wire.
If you are going to use a PWM controller then you don't need the larger wire. PWM controllers use the panel amps so as long as the voltage at the controller is 14.8V or more so larger wire won't help. ie The typical voltage drop from the 12V panels (about 18V) to 14.8V at the controller doesn't get used, just the panel amps.
Serial panels with bypass diodes and MPPT controller are generally more tolerant of shadows than parallel panels.
