It turned out that the trick to having a smooth flying 7-inch long range cruiser quadcopter is to have a frame with thick arms and big motors with low KV.
This is why iFlight
XL7 was replaced by TBS Source One with 7-inch arms
and Brotherhobby 2507 Avenger v2 1500KV motors motors.
It turned out it was a very good decision. Stiffer arms and bigger motors greatly improved flight experience and most of the vibrations and bobbing are gone.
Finally a smooth and efficient flight...