Just actually watched your vids. Where HUB says you are too upright in behind view, I get what he is saying, but the way I think about the throw you are not upright enough or dynamically upright/stacked/balanced. In the behind view you would be in seated posture with nose behind toes, instead of stacked upright/athletic nose over toes.
Agree with SW ^: more upright in athletic position over toes.
Definitely have not been doing that. But I understand the feeling, coming from a boardsports background. Going to be tricky to do at a slow pace, but I can feel tons of leverage and pump pre-swinging back and forth in standstill.
I haven't been able to figure out how to stand over my feet correctly, which is why I've either been way up on my toes in the X-step or now too much behind the heels. Good to understand it's this fundamental though so I'll try to figure it out.
First, I completely understand the slow paced strategy. I've been doing the same slow paced throw for the past year and it has helped me immensely to nail down the mechanics. I'm sort of cherry picking one of your quotes but just saying that I also get the struggle of how to remain slow and in control yet over the toes/not flat footed. At this point, with your knowledge and ability, I think you could afford to take a little more nastiness mentality into the shot and carry more momentum into it. You have the understanding to do this and yet still maintain discipline in regards to all of the throwing principles.
In terms of "how", try setting up on right foot and over toes at setup/start of the address. Watch how Eagle's body position (and face) says I'm going to absolutely going to crush the **** out of this disc. He's got a "forward tilt" which is what hub meant and/or he's in an athletic upright position with spine weight over toes as SW said. Try setting up all weight/spine over front leg and toes at the start of the address.
https://youtu.be/sNNT1s4gsuk
https://youtu.be/sKWNGjcnepg
Side story, that is applicable, if you care:
So, while learning about form at DGCR, I've been approaching all sports differently and thinking about how I'm doing everything. So, in soccer, although I can use my left foot to cross or shoot in a pinch, I've never been very powerful in comparison to my dominant right foot. So analyzing one-two step shots with both feet, I noticed that with my right foot that all of my body weight and spine were completely over my right foot at the address. In comparison, when I shoot with my left I tended to be flat footed and/or with some of my weight and spine still over my right leg. So I started getting my weight and spine over the left foot at the address - and really up to the point of contact - and saw immediate results in terms of power. The other day as I was practicing this a lightbulb went off and I thought, I should apply same approach with disc golf - duh. So, was playing a best ball style with a buddy the other day and we came to a pretty open 550ish hole and I went for it, starting with weight over front leg/spine/forward tilt over toes - I gave my best Eagle impression. Longest most complete distance line I've ever thrown. For real. My buddy said, "woah - I didn't think it was going to do that!" (I lined up right to left stride w/ hyzer flip out to the left, full turn to the right w/forward fade).
If anything starting with more weight on right foot and over toes if anything made crushing the can closed and getting a solid brace more natural/easier. It's going to take me time to replicate this every time and maintain principles and not getting ahead of myself but the results on that hole and a couple subsequent ones suggest that its very much worth pursuing.
Obviously, make sure you still get proper pelvis turn (etc.) and, of course, don't allow more momentum equate to going too fast forward/laterally (I use upwards vertical cross step to slow the forward momentum FWIW).
Hope it helps - I'm not saying anything different, just relaying how the "upright/athletic/on toes" thing really clicked in terms of "how" for me.