Couple things:

Contact patch varies from tire to tire...And that can be with the same pressure. Changing pressure to stay in line with tire manufacturer's intended contact patch really is the correct solution. Thought there are other considerations... Bounce of the tire is matched with suspension travel, rebound rates, springs, etc... The suspension is geared up as a "unit" with the tires mounted from the factory. SUVs have a much bigger variance in tire selection/type than sedan (cars), so more has to be judged by the driver and performance. In general, though, I agree that starting with recommended manufacturer (VX suggested) pressure is your best option. My 285-50's will take way more pressure than 29psi. Even the stock, original tires would take more than 29psi. That should be your clue!

I think there's a ride trade-off vs. MPG. More air is gonna give better mileage by reducing the contact patch (that may or may not be starting at the same area as stock.) Here, it's only a matter of less patch=less friction. That will provide better MPG. But you'll have to judge issues with tire bounce and handling if you stray more than say....10-20%. No one can determine YOUR preference. Softer (until dangerously soft -- less than 20psi for example) should always make the ride softer -- as it will absorb more bumps.

I can't see how 20, or even 29psi, can feel the same as 40psi. There's gonna be some difference. The harder you drive, the more obvious it'll be.

Softer will "roll" more in turns and degrade handling. Harder may degrade traction -- especially in rain/snow.

Pick what's best for your taste, climate, and mpg/longevity concern. Really, there IS a correct pressure that determined by viewing tire wear and contact patch. So, you could/should let those decide for you. If you'be strayed too far from stock configuration, you may just have to compensate and adjust your suspension.