I like the location of the spare tire! I think I would pull out the spare and use it for storage though. Unless I was taking a longer trip. Most times if I get a flat in town, I just take the car to my local store to get it fixed.
One problem with a really long bed extension is that the weight tends to get distributed to much on the rear axle. Might not be a problem with the Rivian, but if to much weight is taken off the front axle, steering issues can happen. We don't know how much weight is on the front and rear axles, yet lets say it is 2,500 pounds on each axle. If you load 1,000 pounds right over the rear axle, that would place an additional 1,000 pounds on the rear axle, and 0 on the front. However with the bed extender, say the load is 10 feet long and 100 pounds per foot, that could mean that 700 pounds is behind the rear axle, and 300 in front. That would place the center of that 1,000 pound load about 2 feet behind the rear axle, thus taking 200 pounds off the front axle and placing 1,200 pounds on the rear axle.
If each axle started with about 2,500 pounds, it would not create a problem. However if the factory truck only has 1,800 pounds on the front axle and 2,200 pounds on the rear axle, transferring 200 pounds off the front axle and now having 3,400 pounds on the rear axle could present a problem.
If the bed extension is used to carry a 6' long motorcycle or other light loads, it should not present a problem at all!