Just based on my own experience with quite a few roo's- I would say that your guy is not too far gone to re-train. However, you need to really ask yourself, "Do I REALLY have the time and energy to devote to making this animal understand what is acceptable behavior?" It really does require a BIG commitment from you to really change his behavior. I could go into all the details of what has worked for me, and you have done the research on what others have done, but you said none of it has worked. The thing is, you have to KEEP doing it, and doing the RIGHT thing for YOUR particular roo. If you are willing to spend an hour or more a day for weeks or months, you could probably have a good roo. But also remember, roo's dont live forever and the time spent may be time wasted.
Unless he is some super-roo with amazing lines you cant be without, you really only have a few options in my opinion- Keep him locked up 24/7, Eat him, re-home him, or spend the time working on him. Really though, there are LOTS of decent roos out there if youre willing to be patient and look for one. Or try raising one yourself and be real sure your kids spend time everyday holding him (that may help, but sometimes it doesnt) the holding and petting needs to be a long-term thing though, which goes back to my point of needing to be really dedicated to taming your roo.
Me? I just let my roos be roos and ignore them for the most part. The one that got uppity with my son is loooong gone, and my new guy grew up here and keeps to himself no matter what we do. Our kids handle all our chooks when they are young on a daily basis, then they stick to holding the juvies or the more docile hens, but the roos never care if we mess with 'their' hens.
Maybe you need to find a BIG but very docile roo who can put your naughty roo in line? I know my big guys keep my younger roos from acting out with almost no fighting. IDK, but I hope you can find the right solution that works for your flock and family.