So switching to rubber tires from NinjaFlex has pushed the stress from driving spiritedly back into the range of snapping plastic axles again. The shear point is at the inner base of the wheel rims on both front and back. The new combined HubAxle strengthens the front but for the rear… I think the best option is to use thicker ends for the driveshaft (beveled version of the main driveshaft thickness instead of a thinner cross-section) and use larger diameter mounting holes in the rims.
If you made the shaft hollow and reinforced it by epoxying in a thin steel rod, you’d get much, much stronger shaft.
Adding rear suspension would also take quite a bit of the shock load off of the axle. (I haven’t broken anything since.)
what motor do you have?