Awe, poor little guy. Thank you for making him a part of your family and giving him the life and love he deserves...Remi's situation is all too common with pet parrots, and no matter how many times I see it at the Rescue or hear about it on here, it still breaks my heart every time. I'm glad that Remi has found his forever-home and this has a happy ending. And for only having him for 8 months or so it sounds like he's made leaps and bounds...It is so difficult for them to trust after they go through the first years of their lives like Remi did, so it's wonderful that he is gradually opening up more and more...And your bond will only continue to get stronger and stronger...
I agree that based on his history, both with his former owner(s) and then also for his long stay at that pet shop, it's likely that Remi is exhibiting Feather-Destructive Behavior, especially since he was doing it while in the pet shop and long before you ever brought him home...However, because he spent so much time in that pet shop (honestly the time doesn't really matter, it could be a day, a week, a month, or a year in a pet shop, especially one that isn't quite up to par and where he had lots of contact with other birds), it is quite possible that he's been suffering from some type of infection ...
***In my own experience, what jumped out at me immediately after reading Remi's history is the great possibility of an external, topical Fungal/Yeast infection. It's extremely common and very contagious between birds, it can hide and grow quickly or grow extremely slowly over the span of months to years, it can be whole-body or just in one particular area, and the number one sign/symptom of it is chronic Barbering/Plucking/Scratching/Itching over an extended period of time...I cannot tell you how many parrots have been surrendered to the Rescue by their owners, solely because they had started exhibiting some type of Feather-Destructive Behavior and it had continued over the span of months to years, and the owners did everything they could possibly do correct it as far as their environment, diet, out-of-cage-time and direct attention time, etc., and even having taken them to a Vet who ran Fecal testing and sometimes even Blood-Work (a lot of the time these owners do everything BUT take their bird to a Vet), and since nothing was found and nothing they tried worked, they made the decision to surrender or re-home the bird...And then the first thing we do is take a basic skin-swab from the skin in the area or areas where the bird has been Barbering/Plucking etc, make a smear on a slide for the external skin-swab, and pop it under our microscope...And there is nothing but Yeast all over the slide. And you can then actually take another skin-swab from an area of the bird where they haven't been Barbering/Plucking etc. and look at it under the microscope, and you might see very few Yeast on the slide, and you might not see any Yeast on the slide!
You have to keep in-mind that the last thing you want to tell yourself or to say is that "I don't think my bird is "sick" or has any type of infection or illness as the underlying-cause to his Feather-Destructive Behavior because he's not acting sick, he's eating normally, he's not lethargic or sleeping all the time, his only issue is the Feather-Destructive Behavior, and that has been going on for months and months, so he can't be sick"...While you can rightfully come to the conclusion that any other type of pet isn't sick with an infection, illness, disease, etc. as the underlying-cause of a Compulsive-Behavior that involves their skin/fur/hair etc., you just cannot EVER come to that conclusion with any type of bird because they all naturally possess the ability to hide any and all outward signs of illness, pain, injury, etc. for extremely long periods of time, and they all will do this for as long as they can. You also have to realize that one of the much more difficult signs/symptoms of illness for a bird to hide is itchiness; it's much easier to hide all signs of pain, nauseousness, headache, breathing issues, etc. than it is to hide severe itchiness. So while Remi isn't picking at the feathers around his vent area, which is good, that certainly does not in any way mean he hasn't been suffering from either a GI Tract infection (either fungal or bacterial, or even parasitic based on the pet shop he was in for a long period of time) or an external, topical infection of the skin/feather-follicles in certain areas of his body, which can be either bacterial, fungal, or parasitic. Most of the time it's absolutely fungal (yeast), and it's external/topical instead of in the GI Tract, especially when it has been going on for such a long period of time without any other sign/symptoms such as diarrhea/runny stools, vomiting, lack of appetite, etc.
***Anyone here who has experience with Feather-Destructive Behaviors will tell you that the very first step to diagnosing them and eventually stopping them is taking the bird to a Certified Avian Vet or Avian Specialist Vet and ruling-out an underlying-cause that is medical/health-related. You can't move forward in treating a Feather-Destructive Behavior until you rule-out a medical cause, which would actually be a very good thing, as when Feather-Destructive Behavior has an underlying medical cause, it's so much easier to stop. I highly suggest that you take Remi to either a Certified Avian Vet or Avian Specialist Vet and you have at the very least a Fecal Culture and Microscopy done, as well as asking them to take a culture/swab of the external skin/feather-follicles in the area as well as from other areas of his body and look at them under a Microscope to rule-out a topical and/or systemic Yeast Infection. They'll be able to see this right away if it's the cause. He should have routine Blood-Work done once yearly anyway so that you can not only check for infection, Anemia, etc., but even more important with birds is to have their nutritional-levels checked and their Liver and Kidney health and function checked once yearly. But in addressing this feather-issue, you absolutely need to have both Fecal-testing and an external, topical swab microscopy done to rule-out a fungal and/or bacterial infection....And if either of these do end-up being the cause, then a course of Anti-Fungals is the remedy, that's it...Also, you need to think about Remi here, because if it does happen to be a fungal, bacterial, or parasitic infection that is causing his Feather-Destructive Behavior, then the #1 immediate issue is that Remi is going crazy from the itchiness over all these months or longer, because he probably acquired any infections in that pet shop...And Yeast infections, no matter where they are located, are incredibly itchy and eventually painful, which is why birds have a tough time hiding them...So you definitely want to rule any underlying medical problems out soon, if for no other reason than to know that Remi isn't dealing with constant itchiness, and if he is to stop it with the proper medication ASAP...