After all trial and error between the hard case and the backpack, I yet again changed my mind since the hard case is just that; no space for extra tidbits etc, What I ended up doing is packing the MP4Pro in its original DJI softcase and placing...
I've flown with my LowePro backpack and Mavic 3 Pro many times, both domestic and international. It goes under the seat in front of my feet. Never had any issues.
The hardcase is small enough and less than personal item size limits 8 in. x 14 in. x 18 in. Besides this is something not eligible for check in luggage due to Lithium batteries.