Welcome!
My ringneck Cotton was 18 months and wanting nothing to do with humans when I got him last year. Just like Ringo, he walked away if I got near him, but would sleep and preen and chatter if I was just hanging out in the room.
It's been
slow progress, but he'll now take walnuts from my hand and just in the last month we've made progress desensitizing a target stick - I can get about 2 feet away while holding the stick without him walking away.
This is just my opinion, but I think ringneck body language is sometimes more subtle than other species. It's taken me much longer to be able to read Cotton than it has with my Quaker. At first with Cotton, I could definitely tell when he was really relaxed - fluffy head, "soft" eyes, sitting kinda squat. And I could tell when he was REALLY nervous - tall skinny posture, flat feathers, big eyes. But I couldn't tell at all when he would very FIRST start to be suspicious - so I know I made a lot of mistakes trying to build trust, because I didn't notice the 10 things he was doing BEFORE he walked away to tell me I was making him uncomfortable.
Ringnecks also, to my understanding, live in parts of the world with the highest number of natural predators around them. I've also read that they are not monogamous and are not allopreeners, so they really are wired to be standoffish and suspicious of anything new. I honestly think the super social, friendly ringnecks we see and hear of are not the norm. Learning that for me was helpful because I felt so "behind" and probably tried pushing Cotton a little hard here and there and set us back over and over.
For building trust, I did pretty much the same things as you - sat by his cage and talked to him. I added a little metal bowl just inside his cage nearest to where I would sit, and then if he was relaxed and happy while I was talking, I'd drop a piece of walnut (his favorite treat - for Ringo it sounds like sunflower seeds would be what you'd want to use) in the bowl. I used metal so that it makes a little sound when I put something in it. He learned really quickly what the "treat bowl" is.
I also talk through EVERYTHING I'm doing when I'm around him. I talk in a normal voice and until very very recently I never used an excited happy tone - he really did NOT like any show of excitement or attention. It's literally in the last couple of weeks that he's started being okay with me saying "Good boy!" - so it was a big adjustment compared to other birds I've met that would do anything for attention and praise. For example I would have a running monologue the whole time I'd change his food/water/cage papers, "Ok Cotton I'm going to give you food now and take this old bowl out, don't worry, I'm not going to touch you, and it's okay if you move away but I'm not going to reach toward you with my hand, I'm only reaching with the bowl, everything will be just fine and it won't be scary" etc etc non stop.
I have a local parrot store owned by someone SUPER knowledgeable and he said to me once that for birds that are naturally more fearful or suspicious, they are going to be more nervous if you're quiet, because we are a predator, and when predators get quiet that usually means prey is about to be lunch.
I had a hard time breaking things down into different steps, because all the animals I've worked with before pretty quickly decided they'd work with me to get treats. Cotton is motivated to get walnuts, but only on his terms. I kind of have to let everything be his decision or he just won't participate.
So it's not "taming" as a big catch all action that we're doing. It's all these separate things - "taking sunflower seeds from my hand" - "being relaxed when I approach" - "being relaxed around a target stick" - and even though success in one area can build on another area, Ringo might react totally differently to one thing than the other.
Our progression has looked like this -
Cotton would take treats through the cage bars while I was seated, and retreat to eat them.
Then he started coming back to the cage bars "asking" for more.
Then he started taking treats from me while outside his cage (on top, his preferred place to be, always) and would walk fast away to the opposite side to eat it.
Then he started taking treats, walking all the way away, then coming back to ask for more.
Then he started taking treats, walking only a couple steps away.
Now we're working on him being ok with a target stick in my hand.
So the first thing I worked on by itself was getting Cotton used to what "training time" meant - for me I wanted him to know that training time means I'll get close to you and nothing scary will happen.
At first his reward was me STOPPING giving him attention. He wanted me to leave him alone, so that's what I used to help reinforce him staying calm.
I started across the room and started talking, like "hey Cotton, it's training time now, we're going to work on how we communicate to each other and I'm going to work on getting better at knowing when you're relaxed and when you're nervous. I'm going to walk a little closer now" and then I would take a couple steps. You want to get just close enough Ringo is on alert, but not so close that he moves away. The second he looks even a tiny bit alert or nervous, stop. Then wait (while talking softly and looking away now and then so you aren't staring) until he relaxes again. As soon as he does, say "good boy" or "yes" or whatever "marker word" you want to use, then take a step backward or turn around completely.
The goal is to teach Ringo that staying calm gets him what he wants.
If you can get close enough to drop treats for him in a bowl, you can add that step before moving away again.
That's where I would start, anyway, just work on Ringo starting to learn that he doesn't have to be on high alert every time you're moving around. It took about 6 months for Cotton to let me come right up to the cage and offer him treats from my hand, so I think it's great that Ringo is already doing that even if he's super nervous about everything else happening.
For food and toys -
Some birds haven't learned how to play with toys. It's expensive at first for sure, but you might have to keep trying different toys and put sunflower seeds in them to try to get him to investigate.
Most ringnecks love foraging for their food, so once he's eating a more varied diet you can definitely use foraging to get him interacting with more things around his environment. You can start with putting a few toys or crumpled pieces of paper over top of seeds in his bowl, so he has to move the paper/toys to get to the food.
If he likes all seeds, take all the sunflower seeds out of his mix, then when you feed him veggies you can sprinkle a few seeds (minus sunflower seeds, save those for training) on them to try to get him to accidentally eat some veggies.
You can try doing veggies in all different ways, too - Cotton likes it best when everything except sweet peppers are chopped really small. He HATES broccoli or anything green so now that he'll eat pellets and try more new foods (after months and months and months of only eating peppers and seeds!) I use a skewer and put slices of peppers in between green veggies and green leaves like spinach - and he does take bites here and there.
Only recently he's started trying new foods if I put them in his bowl, but most everything still gets tossed on the ground. I think he trusts that I'm offering what *I* think is food now, but he thinks my taste sucks haha
I'm sorry this is SO LONG!!!
But I really do think that what you're going through right now is VERY common with ringnecks! I've definitely met way more people that have been in your shoes, than people who have friendly talkative ringnecks.
You can also pretend in your head that he just came home today - that has helped me on days where I feel like all our progress disappeared. Kind of resetting my mind to start from scratch and not try to make things happen too fast.
Hang in there, you already have enough patience and willingness to make it work, and you both will get there