On the last point, a better comparison would be base 6 or base 14.

10 = 2 × 5
6 = 2 × 3
14 = 2 × 7

Or maybe a better way of thinking about it is the percentage of numbers that divide nicely in the base, as a percentage.

Base 10 has 2, 5, 10 = 30%

So maybe base 3 is the closest, at 33% of numbers being easily divisible.

Either way, 7 is a significantly worse base than 10.