Yes, it does seem that they are 'picky' but it's not so, it's only that they only eat what they are familiar with and trying new things are not 'their thing' but it can be done - and I know this for a fact because I have switched -literally- hundreds of birds (I had a bird rescue for 6 years in Pa with an average of 240 birds in it). ALL my birds came to me as adults with 99% of them being seed junkies and they are all eating a very good diet now so it's a matter of patience, persistence, timing and presentation. Zoey Senegal came to me when she was 7 or 8 and Sweetpea when he was 11 (and, by the way, Sweetpea is a genius of a bird, he not only has a super large vocabulary, he actually speaks cognitively and has conversations with me) and they both eat a nice range of raw produce and their gloop with no problem - they are NOT into leafy greens but they would eat their raw broccoli, celery and Brussels Sprouts with no problem and, sometimes, their chard, too -especially the red one. They are not EXCELLENT eaters like the quakers, the caiques or the amazons but they are pretty good. The trick is to eat with them (their food, not ours) at dawn and to serve the healthy food in the am, saving the seeds for dinner, only. Sometimes, you need to feed the raw produce off your hands until they get the hang of eating raw produce and learn to like it - and, sometimes, you need to keep on doing this for the rest of their lives (Sophie Gray is like that, she will only eat raw produce if I hold it in front of her face - and Linus Too only eats certain things -like zucchini, for example- if I do this while he would eat other stuff on his own so you need to kind of 'adapt' the 'eating arrangements' to the individual bird). My birds don't like pellets either and, in truth, they are not the best dietary option for them, in my personal opinion (I've done research on their natural diets for over 20 years) but they all love their gloop (which one could say it's a fresh, wet, deconstructed pellet). Try it and see what happens. Even a little improvement in his diet will be of a great benefit for him.
As to introducing a female... I was thinking about that last night and decided that I should have qualified the statement. First of all, it always benefits a bird to have a mate - immensely! - but a companion also works (I actively strive to give all my birds if not mates, at least companions of their own species or one very close to it). Now, the thing is that, when birds are not used to seeing another bird and they are bonded to their human, they would, sometimes, not want another bird to share at the beginning - but it does happen if you insist. It took years for Zoey to accept Sweetpea. It wasn't anything personal on her part, she simply did not like to share me with other birds (or my husband), and, when he tried to approach her, she would bluff a bite toward him and/or fly away. But he persevered and, in time, she allowed him to perch on the same branch as long as he was on the other end, then get a little closer until, one day, she allowed him to preen her. They have been sharing a cage now for years and, every breeding season, he makes a 'nest' for her which she examines and examines and examines inside and out (it's the cutest thing!) until I switch the almost completely chewed up cardboard box for another and the whole process starts again. They don't breed (he does not feed her in her beak so she doesn't produce eggs - at least, so far) but I allow them to go through the motions because it's healthy for them and gives them happiness. But, if your idea was for the new bird to be living under the same conditions as yours is now, don't get another one because it won't benefit either one of them. The idea was for you to improve your bird's life and not just get another one. We used to have a member that had one bird and, like you, she could not keep it under the right conditions because she also worked full time outside the home so he started acting up. She came here and asked questions, got replies and went and adopted another bird but the only change she made was to 'train' them in the early am and did not change any of their living conditions... Result? She ended up with two birds that were acting up so she gave them up to a bad sanctuary (an outdoor one in a hurricane prone area!) - did not even try to find them a good home. A real bad ending to the story and the poor human-imprinted birds that ended up without a human of their own!
Just one more comment: birds do not 'get used' to being outside a cage, they were created for it. They have no choice but to try and get used to being inside a cage - if they get used at all because, let's face it, getting used to living without freedom is not an easy thing to achieve... It's the fact that he is caged way too much that is causing his plucking so you would not be making things worse by allowing him more freedom, you would be giving him what he needs. It's like saying that it would be bad to allow a child to play because it could get used to it - it's what they are supposed to do! Birds were not created by nature to live in a cage and they suffer terribly when they are confined - especially if they are all alone which, again, is not what nature meant for them.
Look, keeping parrots healthy and happy is VERY difficult! People get babies and they work out just fine for a period of time -years, mind you! because parrots mature slowly, like people- but, eventually, the bird starts having what people tend to call 'undesirable behaviors' out of the blue and people don't know what to make of the situation. Only they are not really out of the blue, the problem had always been there and it had been 'growing' for years until it shows up with a doozy. This is what happened to your bird. So you came here to ask about his plucking which shows that you love and worry about him (and kudos to you for that!
), in reply, I can only share with you what I know about birds, why they pluck and what to do to correct it but it is up to you to make the changes or not to improve his life enough so he will stop. There is no silver bullet, no magic potion, no foraging toy, no nothing that is going to make him stop plucking. People will tell you to bathe him, to put a collar on him, to get him this or that toy and even to hold training sessions with him but it won't work. The ONLY thing that will is for him to be happy and, for that, you need to change his life so, again, I strongly urge you to start considering a big change in your husbandry for your bird's sake. The poor thing is NOT happy with his life and he has, pretty much, reached the end of his patience 'rope'...