If you were not already frustrated and aware the behavior is a problem, and that it is too much for you and you don’t want to continue dealing with it, you would not have asked the question you did in the title: is it time to part ways?
The behavior you have reported should be scary to you, because it’s dangerous, and it is very unlikely that it will ever go away completely. You clearly didn’t expect everyone to tell you this; that it is unethical to pass this horse off to someone else and the only ethical way to part was to euthanize.
I have had two versions of your horse. One is currently in my barn after he decided it was better to try to kill himself than be ridden. When he’s not in pain/worked up he’s lovely to handle on the ground. There are sometimes though that he is just better left alone and handled by opening gates and letting him barge through and throw the hay and feed on the ground by the fence. He is much better behaved on the ground than your horse. Under saddle he has seriously hurt someone. He’s done it all, spin/bolt, bronc, and rear. Now that he’s home where all that happens is that he goes back and forth between stall and pasture and eats he is calm, but I can still see the potential for the behavior crop up, so the moment that changes his date has come.
The other horse I put down. She reared, she kicked, she struck out, she bit. We went from crazy to calm and eventually would meet me in the pasture, let me halter her, scratch her, love on her. But, it’d better have been a good weather day or she could have killed you. One could barely handle her on a bad day. She threw a twitch across the vet room once…while sedated. Basic care was very difficult to get done and you had to be a very good read and go slow. Again, good enough and very “loving” when there was no serious pressure and with people she trusted when the conditions were perfect. She would choose to rear and flip rather than be brushed by anyone else. A trainer got on her once, she made it clear she’d go up and over so they got off quick. She was put down very soon after that.
You do not need to feel like you are “washing your hands” of an unpredictably dangerous horse by choosing to euthanize. But whatever you choose to do, it should never include riding a horse with an ingrained rearing habit.