i have tried BRD_BOOT_DELAY too. It didnt work.
I have also found that it works with Arduplane 4.0.2 on Cube Orange but doesnt work on future updates.
I have also tested on Cube Black and other version of pixhawks Arduplane 4.1.1 and it works
On the param, I did compare with both Black and Orange, everything setup the same.
Video below shows that it works with 4.0.2 https://youtu.be/77dHUFz9dKc