"drop the I-Frame interval to match (or go below) the camera's frame rate" as discussed the I-Frame interval is automatically managed by the camera. I cannot change it. Anyway the camera chosed to set it at 15. I'm recording at 12 frames per second, so we are not too far from your suggestions.
"experiment with thedigital noise reduction setting. Drop it to 20 from the default 50 setting and monitor the change.". I run this test but nothing has changed in a significant way (still a lot of disc consumption)
Considering I was not making any progess, I decided to move from 3840x2160 down to 1920x1080P. Surprise-surprise, alla the cameras are using the same amount of space (8GB per day). The next step is to revert back to 4k and check what happens with 4k. Please note I didn't change the firmware version.