I've been going thru a similar experience so I'll share the details maybe it can help.
Blood work, stools, every test was normal. I knew by how she was acting etc. that there was definitely something wrong so I pushed for more tests & those results were not normal at all xrays/ultrasound showed massive infection, liver enlargement, renal abnormality & heart disease. My avian vet was floored and consulted multiple specialists, avian vets all of whom could not understand why the blood work/tests did not support what they were seeing. They immediately implanted a Deslorelin implant to shut down her hormone cycle (too dangerous for her to cycle w/the risk of blood clot/stroke/heart attack) & 2 weeks of meds to address the infection. Retested after that & zero improvement.
They felt the next course of action would be to do an endoscope and collect multiple biopsy samples (airsacs, liver, renal etc.) both liquid and tissue and to basically get a visual of all of the suspect areas. They did reach out to other avian vets & schools to review the results & her case and the consensus is an endoscope. Sorry I just got the call yesterday and don't have all of my terminologies down pat yet. I will have a consult a few days prior and have all of my questions etc. ready for them

. I have scheduled the procedure. I know for me it'll run between $1500 - $2000 depending on how many samples, time (they have 3 hours blocked out for the team but are optimistic it will be about 1 1/12. They will go in @ 2 different entry points. I'm leaving it open for them to do what needs to be done since I want them in once - get EVERYTHING they need because it is dangerous & I don't want to have to repeat.
if they haven't done any imaging xrays/ultrasound I'd ask about it - maybe there is more than meets the eye besides the test results you've already received.
Gracie doesn't "appear" sick, looking at her & not knowing better you'd see a healthy bird - which is far from the case.
Gracie's voice did get much lower/gravelly when she was on the meds & that's when she started to use a "whisper" voice to talk to me - but in her case it may have just been a behavior thing not an actual symptom.
They did emphasis that it is a dangerous procedure for Gracie (the airsac entry especially) but in reality we are out of opinions and the lead director/avian vet that is doing the surgery is comfortable/experienced with the procedure.
It is beyond a doubt one of the hardest decisions knowing there is a danger either way.
For me I would ask the details of the procedure, entry point(s), what they are looking for, are samples being collected, which types & from where etc.
Needless to say the last 5 weeks have been a fortune just in vets with no end in sight in the immediate future. That's a'right tho we just call her Misses Bentley as her nickname now. And like Salty said - what better to spend money on besides a life.