Cheers mate, this fixed my issue with Gunfighter MkIV, except the ministick axes had different IDs (axis 3 and 4) which was expected.
Your youtube video helped big time too!
I tried to do the same for STECS as it was getting picked up as controller, but after disabling rotary x/y it stopped acting as controller, and it stopped being picked up in BF6 as input device. This makes me think that BF6 itself is limited to 1 joystick only and so far I haven't find any better way to have it all directly binded in the game rather than using emulator.