Comment by energywut
It's heavily pine forested mountain areas. With a 65-75% overlap, the SFM algorithms struggle to produce sufficient details. Additionally, because of the verticality of the terrain and very tall pine trees there's a need to have multiple angles to generate a good orthophoto. So the grid is denser than other environments for a reason. I'm continuously updating my flight plan based on the results generated -- squeezing density up/down based on observed results.
The 4 hours is an overestimate, it's probably genuinely closer to 3 hours flight time.
I have a similar situation with a smaller plot of timberland. Sometimes even with 90% overlap it can’t stitch. I get your pain.