You are correct, in my searches what keeps coming up is ANCOR 6 gauge TINN coated copper stranded wire
its not cheap around 5$ a foot for the red/black and wire.
From what I have read going with undersize wire reduces the efficiency of your motor.
I removed a 65lb thrust 24 volt Min Kota that always seemed under powered for my boat.
I figured that I would not have to replace the wire from my batteries only jumper going to the 36 volt system.
I looked at the wire and it looked to be 10 gauge. So I kind of wonder how it would have performed with the correct wire.