The reason to want an 8-speed cassette hub is not the axle strength or number of shifts, it's the overall range. 11-36 is within reason and maybe more... depending on ground clearance in middle gears
The crank lengths seem pretty appropriate most of the time, 4" for preschoolers, 5" grade-schoolers, 6" for tweens, and 7" for grownups... give or take
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Genesis 49:16-17