I have several random thoughts:
First off, if you were to do an 4WD setup, each of the wheels/motors should see less load/need less torque.
You should be able to use smaller motors with 4wd than with a single wheeled drive.
So, if thats the case, how small could you make the motors and still have the setup be functional?
Could you do hub motors, but with small enough hubs to leave you with enough room to have an actually substantial amount of urethane? Or could you get hub motors small enough to use smaller wheels, like 60mm?
Second random thought, its not a question, just a standalone thought:
Carbon Nanofiber Inductor Axial Flux Motors.
Just stop and dwell on that for a moment
An axial flux motor seems like an ideal candidate for 6"/8" pneumatic wheels, especially for a side by side direct drive solution a la carvon powertrain:
W/axial flux motors instead of a long narrow cylinder for the motor housing, you could get a short fat housing instead. So say inch and a half long, w/ a 5" diameter. It would get rid of much of the need for wider axles.