Welcome to the forum and I am very sorry you are having such problems with your bird but, in all honesty, I am really surprised that a cockatoo, and especially a LSC, is so high-strung! In my personal experience, there is no more outgoing species than the Sulfur Crested! They are all hams and enjoy people so much that they even like children - which is saying A LOT when it comes to parrots!
Without being there and observing him in his daily routine and interactions, it's almost impossible to determine what the problem is and has been for the last seven years... which is way too long for a bird to live in a such a constant state of high anxiety. Because even high-strung birds end up relaxing after a while if what is causing the anxiety disappears. It might take a long time and it always implies a long period of observing and tweaking cage, routine, etc but, eventually, one finds the 'just right' environment and routine that makes the bird relax.
So, because I can not tell why your bird is the way he is, I will tell you what, in my personal experience works. I don't know what his normal diet, light schedule, routines are, etc so if there is anything I mention that you are already doing, simply skip over that part.
What I do is:
1) put them a super strict solar schedule with COMPLETE darkness at night because a messed-up endocrine system means an overly hormonal bird which, in turn, means a bird that is in chronic discomfort and even pain -a bird that has been super horny for years and has no relief in sight is not only in pain, it's also TERRIBLY frustrated, this causes anxiety.
2) change their diet to an adequate one which means no free-feeding protein food (it makes them produce sexual hormones) and, in the case of a too, A LOT of the right food and raw produce (because a bird that fed an inadequate diet will end up either hormonal or deficient in nutrients - I'll give you an example, a high-strung bird benefits immensely from a diet that has foods rich in Tryptophan which helps them produce serotonin, the happy hormone.
3) they need more company than other birds. The truth is that parrots should never be alone and this is not my personal opinion, it's the way they evolved to live and cockatoos are much more needy than other birds so a cockatoo that is left alone for hours and hours is going to be a stressed out bird - no two ways about it.
4) I also, like you, leave them alone until they take the first step to approach me but my case is different than yours because my high-strung birds are always new birds that were rehomed to me.
5) they benefit immensely from a super strict and never changing routine. This, I believe, is what one would say it's 'my secret'. People often ask me what is my secret, how do I manage to keep so many birds that have all these different 'baggages' happy and, in truth, I don't have any. I tell everybody what I do but I think the main difference is what my children call my OCD
I am very particular about exact routines, schedules, etc. Every chore and routine is done exactly in the same way, every single day no matter what day it is or what is going on in my life - but I am quite the maniac when it comes to my birds... To give you an example, the following morning after coming home from the hospital, after having a complete abdominal hysterectomy, receiving radiation treatment and getting morphine for the pain and even thoug I was supposed to stay in bed for 7 whole days without getting up at all, I did not only get up, I went down the stairs and did all my birds before anybody got up because I was so worried that my birds had gotten anything done 'the right way' for the three days I was away. I firmly believe that the unchanging continuity of care destresses parrots in captivity.
Then there is the observation of the individual bird and the tweaking and tweaking. I'll give you an example: I recently took in two female IRNs, both of them stressed out from a human light schedule, a diet too high in protein (they were free-fed seeds) and inadequate care (these birds never came out to fly). Their previous owner loved them but she did not know enough to care for them properly so one of them plucks and the other one is terribly high-strung. The one that plucks is getting much better! Peaches no longer plucks under her wings or her rump and is allowing most of her chest feathers to grow back so we are good there. The other one, an albino named Casper, is taking longer because she is a nervous Nellie big time so I've had to go very slwoly with her. To illustrate this point: it took me two weeks to figure out how to cover their cage at night without her freaking out and it takes me a whole 5 minutes to accomplish this because I have to move veeeeeeryyyyyyy sloooooowlyyyyyyy, repeating the same phrase over and over and over (Nite nite, sweet dreams, I love you) while I cover it in stages and stopping in between each so Casper has time to 'adjust' to each step before I start the next one. But I figured it out and I can now cover them without her having a fit where she bangs herself against the bars. She is also getting better, she now steps up, throws kisses, does what I call 'balconear' (it's a Spanish verb that means enjoying a balcony and what I call it when they perch at the door of their cage without actually flying out) and even comes out of the cage.
Now, what I would urge you to do is to re-evaluate your husbandry in detaill and as objectively as you can and see if you can follow or improve on all these steps but (and I don't mean to criticize, I am just pointing out things that stood out to me on your posting and which I might have misunderstood so, if I did, I apologize in advance) putting a single cockatoo in a room doesn't work. I know that you were trying to give him a large safe space for him to roam and this is truly a wonderful idea but cockatoos never do well on their own, they need company. It'ś the way they are. And bringing in the guy he hates to make him do things is not a good idea, either.
I hope this helps you and your bird and, if there is anything on which you would like me to elaborate or clarify, please let me know.