I'll ask two questions just in case you hadn't considered all the options. If you've already ruled these out, no problem:
- Could you tolerate having the camera mounted lower? (Under the door on that flat panel)
- Could you tolerate having the camera mounted off-center? (to the left of the door hinge)
With respect to how high the camera needs to be: I have two cameras on my truck - one replacement for the OEM backup camera that's mounted under the flatbed tray, (and slightly off-center due to the placement of a center-support), and one that I've mounted at about head-height on the rack that goes across the back of my flatbed camper. I use the upper camera as a rear-view mirror, rather than for backing (with an always-on display mounted in the rear-view position), therefore that camera is aimed "higher" so I can see back down the road behind me on the highway, etc. Whenever I'm backing up and the "low" camera comes on with the OEM backup display in the radio, I have a more-than-adequate view of the area directly behind the truck. I definitely do not see a need to mount it any higher for "visibility". Similarly, with the slightly "fisheye" nature of the lens, I do not notice that the lower backup camera is mounted slightly off center.
If I were you, I'd temporarily connect all the wiring and enlist the help of an assistant to hold the camera in various positions and angles on the back of your truck. You can also setup cones or water bottles in the key positions so you can try to spot them on the camera view. You might just find that one of the more convenient locations works perfectly well, without the need for messing with mounting across a moving hinge.
If you want the perfect "gods-eye" view while backing up, you might also find that mounting
above the door gives a good view - so get your assistant a step ladder!