Horse has your number. And won't make a bit of difference if you sell this one, and get another, or get another 20. The problem here is you, more than the horse I think.
While you are of the "won't hit the horse" fixation, "hitting" the horse does not mean flailing away with a 2x4. It can be a firm tap, as a correction, which I can assure you will not hurt the horse near as much as a corrective tap from another horse would.
And lunging per se will do nothing to build respect. If owner/handler does not have the dominant role, horse will behave just as badly during any exercise in the round pen. Seen it happen too many times, and every time, it is the human's fault.
The horse wasn't listening to the hackamore before, and he is not refusing to listen to it now. In fact the hackamore had nothing to do with any of it, the horse was going along with the program while he figured out whether you could control him or not. Once he had a handle on that, and found out you couldn't? He has begun doing exactly whatever he wants to do, because he knows you won't stop him. He would do the same in a halter, or a spade bit, or a harness for that matter, hitched to a hay baler. He is running the show.
The "respect" everyone talks about, does not come from gadgets as much as it comes from the handler being the boss of the group. This needs to be shown when haltering, leading, saddling, grooming, or riding or feeding. And even when handler walks through field, that leadership needs to ooze through the pores of the human, as it does through the lead horse in the pasture.
The leader of the herd, or the upper pecking order, does not worry about bonding with lesser horses, and they do not care whether the other horses like them. And they don't worry about hurting the other horses either.
I would expect that this horse has been showing you all along signs that he was becoming more and more dominant, and you just haven't known enough to identify it. Pushing against you, rubbing head against you, snatching head down to graze in hand, or under saddle perhaps, ear pinning at feeding time....the list goes on and on as to the ways that a horse can show disrespect for a human. Even something as simple as "vacant eyes" signals that, just as it does in a child. Eyes focused elsewhere is a blatant sign of "you don't matter to me" and "I will do what I want."
That is why I don't unhalter if horse has "distant eyes" or is looking to see if feed is in, or is trying to see over stall partition. I shake halter and tell them to pay attention to me, and don't release them until they do so.
It is not the training or lack of it. It is your horse handling skills that are the root of this more than likely. If those don't change, you will never get anywhere with horses. Occasionally I have seen a horse that won't push the envelope with a weak handler, but in 56 years now, there has not been even 3 of those. And those horses were just laid back about everything to begin with, and didn't care where they were in the hierarchy period, with horses or humans.
Toughen up in your handling of this horse. All horses for that matter. Act like a leader, and forget being a friend. Horses don't think like that. If there isn't a leader, they will take that position themselves. Cut out any extra talking to this horse, as that makes it worse. Keep things business like, and short of words. And no baby talking, or petting and praising either.
When horse looks at you with the look of "I am waiting for you to tell me what to do" then you will have headed towards a partnership with it, but not until then.
Horses make me a better person.