The Rebel comes with E-rated tires (80 psi max cold) compared to the other 1500 models.
My ORG truck came with C-rated tires and I swapped to an XL-rated tire. Still 50 psi max cold and I don't run them below 40 psi.
My last truck was a Ford F-250 with E-rated tires and I never ran them below 60 psi. I prefer to maintain closer to load capacity of the tires (and also don't like all the sidewall bulge at lower pressures either).
An E-rated tire is rated for ~3400 lbs @ 80 psi. At 55 psi that drops to ~2600 lbs and at 45 psi it's down to ~2300 lbs.
Do the Rebels have more than 3900# FAWR and 4100# RAWR?
I have E rated tires on my 1500. For the GAWR of the truck I run them at 34 and 30psi according to the tire manufacturer's load chart compared with the stock tire pressure and rating. They are rated at 4080# at 80psi.
Ram chose a ridiculously high pressure for the weight of the truck resulting in a harsher ride than necessary. That's not an opinion. It's the facts according to the TRA load charts that are standard across the industry.
An LT 275/70R18 Load range E is rated at 3640#. Not 3400#. According to a load chart, 40psi in the front and 35psi in the rear is sufficient for the 1500 GAWRs. 40psi gives you 2270# per tire and 35psi gives you 2070# per tire.
Your load rating assumptions are incorrect but giving you the benefit of the doubt, there is zero reason to have 5200# of tire capacity with a FAWR of 3900#. A diesel 3500 SRW weighs 5000# in the front. 2300# in the rear is better but still 500# higher than the RAWR. Stock 3500s do not have tire pressures with that much overhead.
Here are the charts. Even though this is on Toyo's website, these are the same exact charts used by every manufacturer for every tire size.
Also, considering your post above about running significantly higher than recommended pressures giving you more "capacity." It also decreases traction and handling due to less conformance with the road. If you need more capacity, buy an HD truck.