WOW very interesting build.
The first design is screaming for an ATV axle like this.
this is just a picture of a chinese ATV axle as an example that I found just to show you.
I doubt you would be able to find an ATV rear axle assembly like this there though.
The thing about that axle is it is hardened steel and is only supported by 2 bearings in the center and has the hubs, sprocket and brake disc all there.They really are about as simple and compact as it gets.
The bad thing is if it breaks you'd need to get parts for it so building your own is really the best way.
Excellent builds I love reading about your progress and solutions to the various problems.