Here is my concept for a 6 wheel drive 4 wheel steering Wheelchair. The suspension system is similar to many of the Mars Rover designs such as Curiosity and Perseverance, but I relocated the pivot points lower, and the steering system higher to lower the center of gravity without sacrificing turning ability.
The hand crank is shaft driven through bevel gears to a Nuvinic CVT that chain drives 3 differential systems from a pedal trike system. The differentials rotate 6 flexible drive shafts (Extreme duty versions of a dentist drill attachment) to independent geardrives on each wheel. This allows all 6 Fat Tire 20in bicycle wheels to be driven without binding the drivetrain while turning. The flexible drivetrain also allows for smooth independent motion of each wheel without complex chain or gear systems that would be used in conventional systems. The drivetrain also includes an extra sprocket for torque sensing electric assist motor to help overcome the additional friction of the flexible drive shaft system.
The hand crank arms twist left and right moving 4 heavy duty push pull cables (similar to transmission shift cables) that independently control the steering of the front and rear wheels. The rear wheels counter steer in correlation with the front wheels to maximise turning without sacrificing stability.
The seat back lets down completely parallel to the ground so it is easy for the rider to transition from a standard wheelchair into the 6x6 seat.
This system could easily be modified to 6 brushless DC electric hub motors and electric actuated steering in place of the hand crank and flexible shaft system for improved performance.