UPDATE:
The necropsy results came back - the diagnosis was end stage kidney disease with gout, so there wasn't much we could do. What's worse - there was some miscommunication between our vet and the lab so he was group cremated, meaning we will not get his ashes. I'm devastated, and my significant other is furious.
We were told by his former family back when we adopted him back in April 2020 that he was 1.5 year old cockatiel, however the pathologist seems to think Bowie was a much older cockatiel than what we were told.
Do you think with end stage kidney disease he was suffering for a long time? He acted different around 3 to 4 days before he passed, other than that he seemed happy.