Oh man, I didn't realize how long it'd been since I made the original post!
Apollo is doing really well—to the point where I honestly don't think he'd even be happy returning to aviary life. He's still not the snuggliest bird ever and interaction is always on his terms (which I was prepared for from day one) but he's a totally different bird to the one I brought home. He's quiet as a mouse for most of the day and only screams when I get home from work or when he sees something scary out the window. I won't lie, it's been a massive relief. I was honestly close to tears most days early on with how overwhelming the noise was. I don't feel like I have a second full-time job trying to keep him entertained anymore—he spends most of his time playing with toys by himself nearby, then seeks me out in the evening to sit on my knee while I talk to him (which he loves—his cheeks get all puffy and he starts beak grinding and mumbling back to me) before moving to my shoulder to preen himself until bedtime. He still loves his night-time piano! Finally got him to switch to a pellet diet (none of my trickery worked—he just randomly started eating them one day lol) which made a world of difference to his anxiety and has also improved his feather condition—though I'm still only allowed to give him scritches under very specific circumstances, so he looks a bit like a porcupine right now.
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He and Jinx aren't really buddies in the way I'd hoped. They're tolerant enough of each other that I'm comfortable letting them out together and even letting them share a cage for some supervised time outside, but they don't preen each other or really interact outside of chattering to one another when they're in separate rooms. Sometimes they both get bird zoomies and will go mental flying laps around the living room together. Hopefully they keep bonding but for the time being I'm happy, as long as they're not fighting and can provide each other a little bit of company while I'm out.
That's about all I can think of in terms of updates! Overall he's definitely much more independent than a handraised bird and he doesn't tolerate being pushed (or bribed) out of his comfort zone, but it's really rewarding whenever he does trust me enough to try something new.