I just improved my kid's technique. You can see some of what I'm talking about in this video (even through it's not slowed down). I tried to get him to flick his forearm behind him as his shoulders become fully loaded to ensure that the arm is in the optional position to be bent back as he begins his forward rotation.
In my very humble opinion, starting with your arm outstretched is hurting you. That arm position contributes nothing to the desired movement, and it only serves to distract and prevent you from achieving the timely and ideal arm position. You'd be best served to keep your arm close to the 90-degree angle from when you break to when you are set to deliver.
There are guys that can start with an outstretched arm and still deliver with good velocity, but their sequencing and timing is good, and yours is not.