Big grain of salt, but I've seen you mention stiffness previously, and I can see that generally. Something I've added is some easy stretching before practice, not so much to loosen up, but to get my brain connected with the hips. I just sit on the edge of a chair, sit up and get my posture straight and open my legs at 10 and 2 o'clock or so, cross my arms on my chest, and lean forward while maintaining that posture. That activates those hip joints and gets me thinking about that feel in an aware way.
Then I do swing drills, like with a hammer, but just feeling the weight of my arm, it helps me to feel that hip joint at each end of the extreme extension. I get my breath in tune, maybe with an exhale at the deep 6 o'clock position and peak of inhale at the fully extended positions. Now feel that zero gravity moment when your arm is out and let it hang there before it falls. Focus on that feeling of it hanging, weightless in the air. When I'm doing this right, I may have to exert just a little force to keep my elbow from bending and my arm falling back towards my center. At that moment, the corresponding hip joint should be pretty maxed out. Now when I throw, I let the disc hang in that weightless position and lag the rest of the motion, but that weightless feeling is something I've keyed on that helps me.
When I watch your video work, it looks like you are physically stopping your arm at the top of the swing and then changing it's direction. I tend to go for that feel you get at the top of a quick roller coaster when you rise from you seat. Weightlessness.
Now I suck at disc golf and my form is nothing to emulate, blah, blah, blah—but if that helps...