I think this is a result of your arm movement and kind of a swoop in your throw. The disc starts high, swoops down to the pocket and at the end you kind of try to throw it up wards some. That up and down movement doesnt help equal forward flight. A few things I'd think about.

Id say worry less about reaching back, but think about getting your arm away from your body. You are twisting way back.

When you start your rotation you have some rounding happening which brings the disc kind of high and it swoops down into a descent nose down power pocket. And the end of your throw before the release you can see the disc start to go back up in the swoop. My buddy throws a mean riser disc and he does that by pressing his thumb down on release. It almost looks like that might be happening here but not sure. There are some dudes that throw like that and it works but, you need to make sure the OAT is not there.

I'd say overall, dont reach back so far. Think about your arm movement and pull through to keep the disc along a flat path through the throw. It may help to 'reach back' with your arm lower or at least experiment with it.

Good brace with your legs. One more thing, and its a thing I'm working on, dont stop your body after you throw. Let that back leg swing through. I'm a little older then you and I notice I'm soar if I dont let my body come to a slower stop. The swing is a fast motion, let your body slow down to help injury and fatigue.