I am so impressed with what you've already done! You've done the right things and certainly gone beyond. I think that it is time to talk to someone about it and see if something else is going on.
There is another avenue that you might have considered: giving up. Declaring him graduated from potty training and giving him a supply of diapers that he may use while in the bathroom. Let him know that he is to either transfer the poop to the toilet and flush, or to wrap the diaper up and have a special trash can for him in there. Give him a hug and a kiss and tell him to let you know when he needs a new supply of diapers. Have him wipe himself and wash his hands and take responsibility for his bathroom habits like his age allows.
Don't do this in a spirit of exasperation, but one of passing the torch and letting him own his own choices for pooping, as long as he does his little dance in the correct room with the door closed. Don't interact with him about it again unless he brings it up, and then don't give it much attention at all....not even if he starts to use the toilet to poop. No more poopy parties. His potty habits have gotten enough attention for now.
I've had 2 clients with similar situations where this has worked, so I do recommend it. If you've tried this already, then I would speak with a child psychologist
who can engage him in a deeper understanding of what is holding him back.
If I can be of further assistance, please let me know.