Figured I'd update this thread a little with news
Everything is going absolutely peachy. Already in this single month Simo has improved and grown miles and leaps - where he used to flap about and do "parkour spaz" routines in his cage before (him wanting to escape essentially) he has calmed down and now hardly does it all! Once in a blue moon he will still hop about a bit in the morning. Other than that he's doing just awesome. He's started to play with his toys a lot; not just one of them but he seems to experiment with all of his toys. His particular favorite seems to be the giant swing (it's actually meant for bigger birds but I accidentally ordered it) he'll sit on it and push against a cage's bar with his beak to make it swing and move. Very fun to watch

He's also partial to his spiral rope perch that has a bell at the end; he really likes to make it jingle.
He will also calm down in a matter of heartbeats when me or the husband now approach his cage. Not only does he calm down though, but he will also actually hop onto a perch that's lower than our eye level and shift closer and crane his head / observe us. If I then whistle at him he will whistle back

(actually even as I type this he's swinging on his swing and keeps ogling at me ^^)
We're slowly moving to the stage where we're offering him some millet through the cage's rungs. He still seems somewhat shy about this but that's perfectly okay - we're doing things on his terms of course. One funny thing that makes him curious are "shooting noises" in video games. He will hop to the cage's side that's closest to us and the source of the sound and sit there and eye the monitor. Sometimes he'll also make a tiny peep right after a "shooting". I guess we're ultra lucky that he doesn't mind loud noises at all (there's noisy construction going on closeby too and he doesn't seem to care or react at all) nor to loud game / tv sounds, or even the phone ringing. In fact I think he even likes some of them. I had the impression that loud sounds frighten birds; but on the other hand I don't see why the opposite wouldn't happen either.
Food wise we've been experimenting with salads and veggies - he seems to really like just about anything save for sour apples (but to be fair, I don't enjoy those myself either). We give him way more veggies than fruit of course - too much sweets can't be good even if it's just fruit; or so I heard. He seems to favor larger chunks to small bite sizes so he enjoys nibbling.
Bath times also seem to be a hit with him: he seems to love love love it when I gently spray / mist him with a spritzer (pure clean, skin warm water, no soap of course). What I don't know is if it's okay or not to give him short burst from the hair dryer on a gentle setting if it happens to be a cold day. This would be good to know because winters can get very chilly around here and I understand birds can get cold easy if they're wet. Then again hopefully this won't happen because it's warm inside our house, but, this would be good info to learn anyway.
Finding out what treats he likes has been a bit of a confusing experience. He doesn't so far at least, appear to care for nuts. We've yet to try pistachios, macadamias and hazelnuts. So far the other more "common" nuts don't seem to be a hit with him save for peanuts. And we're not too fond of giving those to him :/ His only true favorite treats seem to be millet, dry corn on the cob and organic oatmeal cookies (sugar free fat free etc.). This doesn't thankfully extend to his normal food (he seems to love every veggie & fruit we give him as well as his seeds which we give him twice a week) Strangely enough he loves the dry corn on the cob (especially when we put a piece into his foraging toy) but loose kernels he doesn't like.
Now, despite how everything is going wonderfully we never of course, take anything for granted but at least so far he's been awesome and has certainly become quite a bit more social, especially considering how he's a "wild" bird.

The plans are still on to get him a buddy but this is turning out to be surprisingly difficult because hand fed parrots are not really very popular among pet bird enthusiasts at least in scandinavia. But yeah! So far, so good. It's probably a very very good thing he has constant eye contact with us and can observe our life and activities.
We're currently taking care of my sister's dog too. He's shown a lot of curiosity about him too, though he will flap and squawk if he gets too close to his cage.
