The Rivian has four independent electric motors, zero need for braked traction control or diff locks.
That kind of drive train absolutely obliterates the Defender in terms of capability.
Good size tyres with plenty of float and it should do very well. It certainly won't feel it's  weight or any drag with all that power!
If they can put a gas turbine range extender on these platforms it will be next level!
Mitsubishi Mi-Tech Debuts As Open-Top SUV With Gas Turbine Engine
To keep up, Land Rover would need to completely redesign their platform for FOUR motor drive!