I'm going to suggest a change in strategies a little bit and try to get you out of the weeds. Guess what - I had these problems too!
Everything wrong in your throws is also in the swing drill/stretch (this is why they are helpful when you do them right - you are fixing how you move, not doing drills). Can't tell if you have a flexibility issue yet until we get you moving in this direction in the first place. You're still not getting in balance and you're already tipping off the rear leg while you're swinging back.
1. You rear leg is just sitting there like a stiff post. It needs to function like "
walking it back." Do that Elephant drill walking forward like he is and throw.
2. I think even when you do it, you're still going to have trouble with the kinetics and ground forces. I can tell your body has no idea what they feel like. You are going to need to swing something much heavier than a golf club to get your body to move better and it's going to take you a while before you can do it with a lighter disc. So:
a. Use a 5lb weight in the swing drill above. If you tip like you do with the golf club, you're probably going to get out of control immediately. You need to heave it (carefully/slow at first) back around like SW and let it swing you into follow through in that direction. If you can't control it slow, you can't control it faster.
b. You, my overmuscling weightlifter friend, probably need to get a sledgehammer or heavy bar and do this. No, not metaphorically. If your arm doesn't feel like swinging a sledgehammer like this, and if your body can't do it like this older guy as effortlessly as this, you're going to be chasing ghosts for a while. By the way, great rhythm training and workout for exactly the chain you need to throw far/more effortlessly:
DETAILS if needed:
1. you might not be letting your rear hip coil internally and you're still blocking the action. This is something pitchers do without thinking about it. You'll probably need to attack it head on. Maybe this drill focusing on the rear hip around 6:58:
2. I don't think you have
side bend at all because you're also not stable in the rear leg. You're swinging your entire torso back together. Instead, you need to allow mobility through the upper back. Try this in athletic stance. Difference in your movement is you're getting absolutely nothing out of your legs, so your upper has nothing to anchor your backswing.
Notice this is exactly the same thing as the sledge above.