Here is what i tryed without success ^^;
var q_wanted: Quaternion
var w_ax= Vector3(-1,0,0)
and in my function triggering the tween
q_wanted *= Quaternion(Vector3.UP*w_ax, PI/2)
tween.parallel().tween_property(cam_target, "Quaternion", q_wanted, R_SPEED)
i didn't try much yet but i answer already to explain what i try to do.
I'm using TextureButton for the signal but it doesn't really trigger the movement, it send a path like "WWNE" to a function that make my cam_target move one tile at a time using the tween.
so for this example WWNE, the tween will be instantiated 4 times and move my cam_target west, west, north and east; and i was using tween.parallel() for the rotation because it must be triggered at each step of the move and while moving from a direction to another (if it changes).
i actually changed my function and put it in func _process(delta) because i wanted to try different things with delta but i wasn't successfull either...