Welcome to the forum. We have lots of people dealing with bites, and lots of ways people find success in dealing with theses issues. You've gotten great advice, and I will try and add my take to, for trying to help.
First take a deep breath, this took awhile to get to this point, and it's going to take some time to fix.
The first thing I would suggest, is doing observations. Objective observations, were you try to forget what you think is going on, and try to observe with new eyes. You might be surprised at what you observe. I did this when I had some agression going on in my flock, and I was suprised that my observation revealed different issue than I thought.
Second I think this conure had no home base. So there is insecurities, and trying to control the whole home environment. I would suggest going to Amazon and buying one of those large box style cages they have for around 100 bucks or so. Make sure it high off the floor, I got one by mistake that is much lower to floor than my other ones and the birds hate it. Set it up with a lot of short perches, and only one that goes the whole length of the cage. Set up so he can bounce from perch to perch around the cage. Make one high corner, that is shielded by toys, so he can retreat there out of sight. Then also attach many perches on the top of the cage, and if you like , use the vertical space above the cage , by useing a ceiling hook, and fishing line to hang a bungie or rope hoop, so it hanged just a couple of inches higher then the rope perche's you have on top of the cage. This is now his territory. Start keeping him caged during the day when you are away, and at night. If he is cage agressive, that's ok, it's his cage, just let him out before you mess with stuff. Still have him out when you are home. Do not let the budgie be free while he is caged, or the budgie will land in the cage and be injured. You can get clear arclic sheets at home Depot for around 20 bucks to out on top of the cage, to help protect the Budgie feet if he land in cage while the GCC is inside..
I think doing this will give your GCC a place to call his own, and make the rest of the house neutral.
I would also set up 3 areas in the house ( living room) that at his places to hang out. Pet supermarket and probably online or other stores sell tall metal stands that you can attach big rope hoops to. They are good because you can move them to places you want. One again you can use ceiling hook and fishing line to hang bungie Orr hoops in different locations. Try to hang at eye level. I have two over my breakfast bar, with cheap dollar store trays underneath. This gives your GCC places to fly to, and feel secure. Zip tie some treat sticks, millit and toys that are easy to shred to them. All my birds are flighted, and use these exclusively, instead if random places around the house or furniture. And will help with security for your GCC, places to feel safe.
Start over with your bird, as though it's the first day you brought him home. Spend time bonding to him. Hand feed treats everytime to you go to say hi. Take him on a tour of the places you set up for him. Praise him for useing those places. Reintroduce him to everyone in the house, abdvitget pets. Create morning rituals, everyone's says good morning to the bird when they get up. Everyone says goodbye when they leave the house, and everyone says hello when they return to the house. It allows the bird to feel part of the flock, and respected. It helps provide security on what to expect, things feel more controlled to the bird.
Set aside a time if the day , you think you can stuck to , that is just you and your bird. Try for a half-hour at least. Hopefully a place that is out of sight of your husband and other pets. Or maybe you get up a half hour early and it's just you and your bird. My GCC chose 830 pm as her mommy and me time. We snuggle, and she chats about her day, sometimes at first she stops around and sounds like she complaining about her life, then she is ready to cuddle. If she doesn't get this time she is one angry and mean bird!
Putva treat dish on top of the cage, and one inside. Your husband , and you, always place a " safflower seed" or treat in the dish when you come and say hi. Your husband should do this many times a day. Always use the same phrases like hello " name" . For sure always do this when you come home, and as you leave. Hopefully , go slow, your husband can work up to giving them by hand.
See if you and your husband and you can share meals with the bird. It's a great flock activity, and bonding.
I'm going to look for some links I have. That I think will be helpful. You can do this! I have had some bad times with my GCC and we have worked passed them.
Try and take any hurt feelings about bites, and let them go. This is a communication problem with your bird. And most of us take the approach that bites are our fault, and we need to figure out what we are doing wrong. Parrots always have a reason they are biting, or several reasons. Bribes, apologies, paying attention to body language, providing security, and predictability goes a long way to fixing.
