Car rental advice for cross country road trip

Hi all,

My wife, our 85-lb dog, and I are planning a 5-week cross-country road trip this summer (early July to August) from LA to Philadelphia and back.

The dog will get the 2nd row (with a hammock). Because of dog supplies and luggage, we need the 3rd row to fold completely flat, giving us at least 40–45+ cubic feet of trunk space.
We currently have a dual-booking strategy for Thursday, July 2nd at LAX:

• 7:00 PM: National Emerald Club Executive Aisle (midsize rental, so much cheaper but a gamble on availability).
• 8:00 PM: Hertz backup rental. Deciding between a Nissan Pathfinder (FWD), Hyundai Santa Fe (AWD, same price as Pathfinder), or paying $700 extra to lock in a Volvo XC90 Hybrid. Of course, none of these vehicles are guaranteed, it's just the class of rental.

We live locally and have total flexibility on when and where we pick up. Looking for advice on three quick logistics questions:

1. LAX vs. Burbank (BUR): Does Burbank's National/Hertz inventory even compare for large SUVs, or is sticking to the massive hub at LAX safer to guarantee a big car?
2. The Pre-Holiday Timing: Is Thursday evening right before the 4th of July weekend a recipe for an empty lot? Would it make more sense to pick up the car earlier in the week (Tuesday or Wednesday) to get the best pick before it's cleared out?
3. The Vehicle Dilemma: How do the flat-folding floors and actual trunk depth compare between the Pathfinder and the Santa Fe? Is the comfort of the Volvo XC90 genuinely worth a $700 price bump for a month-long marathon?

Sorry for the long post. Thanks for any insights!