Guess what!?
Birdy is getting better
No idea why or how though - the vets are shocked. I'm assuming this rules out PDD. He's getting some of his balance back, although he still needs to grab the cage with his beak, there's a notable improvement. Sometimes he falls of the perch but now doesn't just sit at the bottom and instead tries to climb back up, sometimes even right to the top towards the opened top piece, which I'm sure is a good sign. He's also chirping, while not as loud as when he was healthy, still probably a good sign. Starting preening a bit too although I don't see it much ever.
A few things though:
While eating is improved (mainly eating pellets and then soy beans as treats), his poop is of a different consistency, and is more runny than it was when he was worse - it also smells wayyyy worse and sometimes splatters out quite a distance and is almost diarrhea-like. Is this of concern or could it be a further sign of improvement?