I am too flying multiple drones with EK3_MAG_CAL set to 2 to get rid of toilet bowling effect cause there was too much varying magnetic fields around my small quadcopters and i just wasn’t able to compensate that enough, though in my case, i am calibrating my compass in-flight in very strict fitness and i have also done compass/motor calibration on all the drones, and after this i had multiple flights and toilet bowling never appeared again, that’s why i suggested same in your case too
But there’s one more way where you can totally disable compass, though this feature is relatively new so not many people have tried it yet or posted much about it on this discussion forum, you can give it a try as well, but this feature is only available in master branch, not yet available on any stable release