It's very difficult to treat and sometimes difficult to pick up with diagnostics. It is very, very common in budgies and they can be carriers without showing any symptoms while others will get sick. i'm sorry you're having such a rough go of things right now
an antibiotic isn't going to do anything. Amphoteracin B is the treatment of choice as others have said. Although it really shouldn't be given by an owner at home because even if a little is aspirated it can cause serious side effects. My doc will also try sodium benzoate in the drinking water and has had success with it. I'm not usually a fan of meds in the water but this really seems to work for our patients.
It may be that your vets have only given you a 2 week supply of some meds because a lot have to be compounded for exotics and they are only stable for 2 weeks. I know this is a frustrating situation, I can only advise you to bring up these frustrations with the vets you have seen because if they don't know you're frustrated or have concerns they can't address them.
I haven't seen any good data that a non-species specific probiotic will do any good for parrots... I don't think it will do harm but just keep that in mind.
A test for psittacosis (chlyamidophila) shouldn't be expensive. A PCR can be done to detect it and sent through the mail to a lab, I had it done for under $40.
The question I would ask of a vet is not if they are avian certified but of their experience with birds. How many do they see, etc. The process to become boarded is costly and extremely time consuming, there are excellent practitioners out there who will do amazing things for your bird but are not necessarily board certified.
