Rivian is going to be dragging its butt all over the trails, like a Raptor does. Rivian has a 4' bed, no? Rivian packaged the bed kind of weird, due to the gear tunnel.
Battery has nothing to do with spare or not, the battery is actually front biased between the axles, if you look at the cutaway, the reason GM did not put a spare under the Hummer is departure angle. Having been off road with Raptors many times, they drag their tail all over the place. The Hummer was designed with a more off road bias. Vehicle design is always about trade offs...