I think Ariel might have almost been a casualty of the pandemic. I have no proof. However, I had to order a different seed mix than my usual. I've settled on a basic seed mix for years with no vitamins or colors added. Higgins Supreme cockatiel. Everyone but the macaw will eat it and its not super high in fat for my already fat amazons. Ariel plucked quite a lot in her youth and then mysteriously stopped...mostly. My seed supplier ran out of my usual blend so I ordered a related blend by the same company but had the colors and vitamins added. Within a few weeks she was nearing the point of me worrying about mutilation. You couldn't see anything from the outside other than her chest feathers were a little thinner than usual. But, if I pushed aside the feathers, she had a couple of small scabs on her crop and chest. They started out looking like the little blood spot that happens after a feather is yanked out but they expanded. And they weren't in places that she had feathers anyway. They never went deep, but she was clearly itchy. I tried more baths, but that seemed to make it worse, which was consistent with what I found when she was younger and plucking more. Shortly after I ran out of that seed, things were looking better pandemic wise, places were starting to open up and I was able to get a bag of our usual shipped. She stopped biting herself within a month of switching back to the usual.
When she was young, Noni juice did seem to legitimately help her a lot. I've heard of a few cockatoos that it has helped and a bunch that it has not. I looked into it this year but my old brand that I'd used previously is no longer produced and when I experimented 15 years ago, there did seem to be variability in the effectiveness of different brands. It does seem to have a therapeutic dose and its nasty flavored stuff so getting the right amount into the bird without force takes some doing. I used to do a teaspoon mixed with the soft food every day. She is a large sulfur crest though so you might not need as much for a goffin's. Assuming that it is somehow an anti-inflammatory, you might just see if your vet can try a few weeks on meloxicam. Supposedly its well tolerated and likely to have much more consistent results than a maybe/maybe not holistic remedy.