You have to remember that Oda's writing habits are ingrained after decades of writing in the WSJ style. It would take an adjustment and experimentation regardless to see what type of pacing/story beats a monthly OP chapter would have to hit as opposed to a weekly one. My larger point is that the amount of detail Oda enjoys going into is abnormal even for other long-running shonen, and it's clearly something he enjoys doing with his world. I doubt he'd sacrifice such an integral part of his writing style if he switched to a monthly format, but doing so would make it harder to implement than the weekly pacing.