I've actually been known to take my measurements to Home Depot and have them cut up the 16 foot trim for me and drywall is easy enough to cut before loading. [emoji6] As you mentioned, if you just have a couple sheets, you can load it at an angle, standing up. The hatch glass on my current vehicle opens separately from the main hatch, allowing me to load even larger items, leave the glass open and have the item stick out the back. For bigger items or larger purchases, I'll have it delivered or rent a truck but when I know it will fit with the seats down it the hatch open, it goes home right away.Even if the seats did fold flat, there's no guarantee an 8x4 sheet would fit I'm pretty sure I couldn't fit an 8x4 sheet in the Q7, even with it's folding seats..
AFAIK the only thing short of a commercial van in the EU that can do this is a Chrysler Grand Voyager (if you want to close the tailgate). With the Q7 you have the option of the roof rack at a push.
Even with my old beater Kia MPV, I have to stand them up on edge to clear the suspension turrets / wheel wells, then drive with the tailgate open