Depending on your weight & riding environment you should customize your electric skateboard drive train to ensure you have enough torque output without over capitalizing. Additional & larger motors are not always necessary.
it’s more about belt longevity, 9mm in most cases is fine (on mono drive) and will work well for most people, but will have a shorter life expectancy, especially if you are heavy & brake/accelerate hard.
the cost of having 12mm is the same as having 9mm. So going 12mm only brings benefits.