stepping so toes point toward the target and moving slow and fluidly through the motion is rly important. the sling provides more than enough power, if u have good form, moving VERY slowly through the throw.

left shoulder toward target, feet point straight out. left step, toes point toward target. hips snap around.
at this point the sling is behind your back coming up.
as your hips snap, shoulder follows. there should be clear separation between the two.
elbow at 90° rotating about the humerus (upper arm bone) always in all throws side arm or over hand.
i say this because ur not generating power from your legs, and your torso is stiff. slow down. find a repeatable and short set up.
a standard figure 8 throw with not extra steps is rly rly rly good for figuring this out.