Hello Erik, Glad to see you have made progress with the wiring.
1.5mm2 would be adequate even at 10 amps. I don’t know how efr215
calculated 0.9 volt drop over a 10ft.length it would be approx 0.33volts.
The longest run is less than 5ft so the v.drop would only be 0.16volts @ 6volts, with the engine running the voltage at the battery is 7.2 volts so the difference would be negligable.If you run a separate earth wire you can ignore the v.drop in this because you have a parallel cicuit through the frame. Best wishes Ted.