For Fantom series is understandable. Because for them the procedure is just "put the drone on leveled surface and do not touch it". Fantoms also have rigid landing gears for it.
But for Mavic the procedure is changed. You have to rotate drone into 5 different positions. And in neither position Mavic is not rigid enough.
For example, instruction says "dismount propellers", but on picture (and in tutorial video) propellers are on. Whereas Mavic actually leveled differently with or without propellers.
Then, IMU calibration procedure requires to put Mavic on its side. But in this position propellers also affects levelling. And even arms are not hard enough.
And in the last position, staying on tail, Mavic is not perpendicular to the ground at all (someone even suggest to lean Mavic on wall).

So, I'm not sure if perfectly leveled surface is still the MUST for IMU calibration for Mavic.

I have never had to calibrate my Mavic but  on all the drones I have built over the years the compass calibration was the one that needed to be rotated in all directions and planes . The IMU needed a perfectly flat and stable surface to be on when calibration was done . Even heavy footsteps or a loud stereo passing by would affect the sensors due to the vibration . Curious to what others say about this .

Out of curiosity is finding a level surface something that's difficult or impossible for you to do?

I've had no problem finding one that's pretty much perfectly level, at least to the extent that my bubble level and eyesight can determine. And since that's what DJI specifies I would recommend following their instructions for the calibration.

P4 pro you must calibrate on all five sides.

everyday of the year even WHEN PIGS FLY!