I'm so sorry you have to watch Bingo deteriorate like this. One thing he has is a lot of love from you and from all of us that have never met him but feel like he's family. As long as he's not in pain your love and affection will make his journey peaceful and when his time comes we will all grieve with you.
I don't think I could put Bingo down unless he was in obvious pain and distress. Maybe I'm selfish, but it was so hard to watch them take Lilibet out of the room knowing what was going to happen, but she had a bad stroke and couldn't function at all so I had to do it. As long as Bingo is getting some enjoyment out of his life, which from what you write, he is, I'd let nature take its course.
Years ago I had a beloved budgie, Buddy, who was slowly dying of kidney failure for over a year at 13 years old. He was getting very thin and when faced with putting him down I asked a human doctor if dying of renal failure was painful. He said it wasn't- that people just faded away peacefully- so I decided to let him go naturally, and he died at home where he felt safe. Perhaps Bingo is slowly starving because he can't process food anymore. They say that starving isn't painful either.
When I think of Bingo, I will always remember that prize winning photo of him on your shoulder, outdoors, smiling at the world like only a joyful parrot can.