It is difficult to say everything with 100 % confidence because you slid forward on the grass and that ruins many things form wise. It makes raising the right ball of the foot for a clean pivot hard and pushing with the left leg at least partially ineffective. So you lacked initial momentum for the body rotation and your hips were just cruising along with the legs not adding anything so it is no wonder that the shoulders weren't firing fully either. One thing can be said fairly certainly. You tried too hard and were in the words of Barry Schulz herky jerky. You punched the arm back to the not longest possible reach back and yanked the arm immediately forward at full power. Strong arming to the max and i can't confirm measurements wise the words of a couple of way more physical throwers than me that they throw farther yanking immediately after planting the final step.
For me it took a long while to get nearly as close with the yank to a late acceleration shot. But i'm not too strong and fast and have way more experience with late acceleration. Overall most seem to fall in the same category with me in that not pulling as fast as possible from the farthest point of the reach back seems to give the longest throws. But i have problems holding on to the discs for a full pivot anyway so it makes sense starting slower limiting speed in the inward movement of the arm helps in holding on to the disc. YMMV based on many things like finger strength, nerve speed and the lack of explosiveness that could force you to accelerate early to gain the maximum speed you can generate. The funny thing is that the guys that do throw far are explosive. They have enough power to keep on accelerating (probably) until the end of the throw which is the goal according to Newtonian physics F=MA. A being acceleration F being force.
What i'd like to you to try is staying more relaxed mentally and in the arm muscles (like fully minus dropping the disc) and don't start killing it with the arm until later and gripping harder later to not have an early slip. Because everyone has different bodies you need to chart which way gives you the most power so i'd try different arm acceleration positions. At least when the disc is around the right pec because that is the starting point to many drills here and a common ground for people to compare their results with yours. Overall i think the more athletic you are the earlier you can start to accelerate.
Flat shots need running on the center line of the tee and planting each step on the center line. Anhyzer needs running from rear right to front left with the plant step hitting the ground to the left of the line you're running on. Hyzer is the mirror of that.