Regarding a 14mm axle, that is not a common size for electric scooters however it is a common size for bicycles. If you can find a 14mm ID x 30mm OD sealed bearing somewhere, then finding a 14mm axle for it should not be a problem.
We do not have factory specifications for the maximum load of the 8", 10", or 12-1/2" wheels. The manufacturers of these wheels do not provide specifications for them. They are typically used on electric scooters though which have to carry the weight of the rider, frame, batteries, and motor on the wheels. The rear wheel of an electric scooter carries a significant amount of the weight since the rider usually sits towards the back of the scooter. So these wheels should be able to carry a decent amount of weight. I would estimate that 200lbs per wheel should not be a problem, and they can probably handle a lot more weight than that.
Is a thicker stronger axle possible with the small wheels, eg the?: 8"/200mm wheels
This next type below might be the right type needed: "reinforced axle (M12, 8.8) for one-sided attachment"
That info is from this link: 'XYZ' Open Source unwelded bicycle frame
And also from this link: Bicycle frame model 'XYZ': unwelded DIY
I am interested in maybe a 14mm axle. I notice that there seems to be only 12mm axles sold with the electric scooter wheels, and are also sold separately on this website.
What maximum load in kg/lb can the 8"/200mm, 10"/250, and 12 1/2"/300mm etc wheels carry?