I have always followed the manufactures instructions as best I could. 1,200 miles seems to a very long break in period for a BMW I6, but what do I know.
I always make sure I don't hold speed constant, once warm, always give it a decent amount of throttle to seat the rings, but just as important, you need to hold a gear on deceleration for good engine braking and decompression.
same thing for brakes, after 500 miles, a good stomp or two to really set them is has never hurt my vehicles.
Lastly, they obviously want you to baby the tranny for a bit, which again is a little odd as that transmission is in a ton of vehicles and has had great reviews and reliability from what I can ascertain. But again, if they don't want me to kick it down, I will try and refrain.
My 2 cents.